You’re standing on a treadmill. The numbers are ticking up. 10 minutes. 12 minutes. You’re bored out of your mind, but you’ve heard that if you don't hit that magic 30-minute mark, the whole session was basically a waste of time. Honestly? That's total nonsense. Most people approach the question of how long should you do cardio like it’s a math equation with only one right answer, but the reality is much more chaotic and, frankly, more encouraging than that.
The truth is that your heart doesn't have a stopwatch. It doesn't know if you're in a $100-a-month spin class or chasing your toddler around the backyard. It just knows strain.
The 150-minute rule is just a baseline
If you look at the official guidelines from the American Heart Association (AHA) or the CDC, they’ll tell you to aim for 150 minutes of moderate-intensity aerobic activity per week. Or 75 minutes if you’re doing the lung-bursting, "I can’t breathe" kind of intense stuff. That sounds like a lot. It’s basically 30 minutes, five days a week. But here is the thing: those numbers are for "general health" and disease prevention. They are the floor, not the ceiling.
If your goal is to lose weight, 150 minutes might not even move the needle. Conversely, if you are a high-level athlete, 150 minutes is basically a warm-up. Dr. I-Min Lee, a professor at Harvard Medical School, has published extensive research showing that even incredibly small "snacks" of exercise—think 10 to 15 minutes—can significantly lower mortality risk. You don't need to block out an hour. You just need to move enough to get your heart rate up.
Why your "Zone" matters more than the clock
Stop looking at the timer and start looking at your pulse.
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There’s a huge difference between a leisurely stroll while scrolling TikTok and a Zone 4 hill sprint. When people ask how long should you do cardio, they are usually thinking about steady-state exercise. That's your jogging, your cycling, your elliptical work. If you are keeping your heart rate at about 60-70% of its maximum (Zone 2), you probably need at least 40 minutes to see real metabolic adaptations. Why? Because Zone 2 is about building mitochondrial density. It’s slow work. It takes time for the body to start prioritizing fat oxidation over glucose.
But then there’s HIIT. High-Intensity Interval Training. If you’re doing true HIIT—the kind where you feel like you might actually see God by the fourth round—you shouldn't be doing it for 45 minutes. If you can do it for 45 minutes, you aren't doing HIIT; you're just doing a hard circuit. True high-intensity work should be capped at 20 minutes. Anything more and your central nervous system starts to fry, your form breaks down, and you’re just begging for a stress fracture or a torn calf muscle.
Fat loss vs. Heart health: Two different timers
If you want to live to be 90, do 30 minutes of walking a day. Simple. Done.
But if you’re trying to shed body fat, the "how long" part gets tricky. Cardio burns calories, sure. But it also makes you hungry. There is a phenomenon called "compensatory eating" where people finish a 45-minute run, burn 400 calories, and then immediately eat a 600-calorie muffin because they "earned it."
To actually lose weight using cardio, duration needs to be balanced with intensity. Long, slow cardio (LISS) is great because it doesn't spike cortisol as much as sprinting does. High cortisol can lead to water retention and stubborn belly fat, which is the exact opposite of what you want. A lot of bodybuilders actually prefer 60-minute walks over 20-minute runs for this exact reason. It protects muscle mass while burning fat stores.
On the flip side, some people just don't have an hour. If you've only got 15 minutes before work, you have to crank the resistance. You have to make those 15 minutes miserable.
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The dark side of overdoing it
Can you do too much? Yes. Absolutely.
There’s a point of diminishing returns. It’s called the U-shaped curve of exercise. People who do zero cardio have high health risks. People who do moderate cardio have the lowest risks. But people who do extreme amounts—think ultra-marathoners or people doing two hours of heavy cardio every single day—sometimes see an uptick in heart issues like atrial fibrillation or arterial scarring.
James O'Keefe, a cardiologist at Saint Luke's Mid America Heart Institute, has spoken extensively about this. His research suggests that after about 60 to 90 minutes of vigorous exercise a day, you aren't really gaining more health benefits. You're just putting wear and tear on your "engine."
Breaking it down by your specific goal
Let’s get practical. You’re likely here because you want a number.
- For pure longevity: Aim for 30 minutes of brisk walking 5 days a week. If you can talk but not sing, you’re at the right pace.
- For fat loss without losing muscle: Try 45 minutes of Zone 2 cardio (walking on an incline or light cycling) 3-4 times a week. This keeps your stress hormones low.
- For cardiovascular "fitness" (VO2 Max): Incorporate 20 minutes of intervals. Sprint for 30 seconds, walk for 90 seconds. Repeat until you hate your life choices.
- For mental health: Honestly? 10 minutes. A study published in JAMA Psychiatry found that even small amounts of movement can significantly reduce the risk of depression.
Most people fail because they try to go from zero to 60 minutes. Don't do that. It sucks. Start with 15 minutes. If you feel good at 15, stay for 20. If you’re busy, do two 10-minute sessions. The science shows that "accumulated" cardio is nearly as effective as "continuous" cardio for blood pressure and glucose control.
The equipment factor
Does the machine change the time? Sorta.
Swimming is a total body workout. 20 minutes in the pool is roughly equivalent to 40 minutes on a bike in terms of caloric expenditure and muscular demand. The elliptical is a lie—those calorie counters are notoriously inflated by about 20-30%, so if the machine says you’ve been on for 30 minutes and burned 400 calories, you probably actually burned 280. Adjust your time accordingly.
What most people get wrong about "fasted" cardio
You’ve probably seen influencers claiming you must do your cardio on an empty stomach first thing in the morning to burn fat.
It’s mostly hype.
While you might burn a slightly higher percentage of fat during the session, your body compensates by burning more carbohydrates later in the day. Total fat loss over 24 hours is basically the same whether you ate breakfast or not. If doing 30 minutes of cardio at 6:00 AM on an empty stomach makes you grumpy and exhausted, stop doing it. Do it at 5:00 PM after a snack. The "how long" matters way less than the "how often." Consistency is the only thing that actually moves the needle in the long run.
Actionable Steps for your routine
Instead of obsessing over a specific minute count, follow this hierarchy:
- Prioritize Frequency first. It is better to do 10 minutes every day than 70 minutes once a week. Your body craves rhythm.
- Toggle Intensity. If you have 40+ minutes, keep the intensity low (can hold a conversation). If you have less than 20 minutes, get breathless.
- Monitor Recovery. If your resting heart rate starts climbing over several days, or you’re feeling "wired but tired," back off the duration. You’re overreaching.
- The "Talk Test" is your best friend. Use it to gauge if the time you’re spending is actually effective for your specific goal.
Don't let the "perfect" 45-minute workout be the enemy of the "good" 15-minute walk. If you're wondering how long should you do cardio, the answer is simply: long enough that you'll actually do it again tomorrow. Get your heart rate up, break a light sweat, and get on with your day.