If you're not getting paid for it, that's pretty normal. It's even fine to abandon unpaid work entirely if you're just not feeling it (though it's still nice to tell the requester that you've done so, don't leave them hanging.)
With paid work things are a little different because paid work is often predicated on a deadline, but many employers/commissioners are willing to give their artists time to sort things out, since it usually means better art for them in the end.