i always spend days or weeks on any project, it is very rare that one comes together in a couple of days, after that time if it still doesn't look right i either save it (to go back to at a later date) or delete it and move on, the last render i uploaded took about three weeks, because i went through about ten different backgrounds before making one i finally liked, but if that background hadn't of worked out, then the whole thing would of been deleted, it was on it's last chance!!

the big guys over at CGSociety average around 4-6 months on a given project. sometimes the end result is a single render.

the best way to approach it i think is to either let the problem beat you, or you beat the problem.
you just have to dedicate the time needed, to beating the problem.