I feel like people don't think they have worthwhile or statistically meaningful answers to your question, and so refrained from posting.
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It really depends on the skill of the artist, as well as the style of the piece (there
are simpler and more elaborate styles) and the familiarity of the artist with the visual elements s/he has to realize (for example, you might have trouble when drawing a vehicle or complex machine, if only because you aren't used to the shapes and 'language' of those objects).
So, you should just try for yourself! Time yourself while doing different sizes and representing entities common to what you want to produce (work in art for games, or draw characters/portraits for people, etc.)
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