Time in relation to art is a funny thing, because the amount of time spent is in no way an indication of it's quality. Sometimes time does benefit a piece of work, but artists can also spend so much time on something that's BAD. Why?
One theory: the vision becomes precious.
Recently my instructors have been pushing for more iteration. Try a composition 50 ways and then figure out which one works best. If you've exhausted all the options at the beginning, then you can be confident in the direction you take and possibly stumble upon something you never would've thought of the first 10 times. This can be difficult if there's a particular vision that sticks in our mind. We think we can already see the perfectly finished piece and we just want to jump in and work on it, and work on it, and work on it... only to realize there was probably a better solution.
BUT I ALREADY DID SO MUCH WORK!... So what? If it's not clear, it's not successful.
I'd rather start over again, and again, than settle for something I can tell isn't working. I'm not saying starting over is always the answer, but it's important to be open to change and explore all the options. Most important is not getting precious about a piece of work. Care about what you do, but know that at some point you may have to let go of what you thought it should be, for it become what it needs to be.
I mostly hear animators say this, but I love hearing about a character wanting to do something as their working. Glen Keane was talking about the end of Part of Your World when Ariel reaches her hand out of the grotto, and he said "I didn't want Ariel to do that... but I couldn't stop her, she did it."
Our artwork has a mind of it's own. In entertainment we're working with characters, and they become real. Listen to your work, and respond.