I know for "daily paintings," the idea is to finish in one session; but it's more important to me to produce something I can live with than to strictly adhere to an arbitrary rule of how long it "should" take. My time varies. Six by six-inch paintings of a simple subject are easier to finish in one session than larger or more complex works. Even on larger paintings, I don't like spending too more than 2-3 days.
I think it partly depends on the goal for a piece: is it to be a quick study or exercise, is it to be merely "good enough" to sell, or is it to be your best effort regardless of the time? Whenever I get impatient or concerned that I'm not working fast enough, I remember item #5 from this post by my mentor, Don Hatfield. "...we want quick fixes, instant results, short cuts, recipes. These are all enemies of great art...Most of us cannot hang with a work until it is right. The mark of a great artist is how long he or she can hang with a painting and keep improving it--bringing it closer to one's initial vision or inspiration." Even Craig Nelson, a master of quick paintings, jokingly says, "There are no bad paintings, only unfinished paintings"--which implies that any painting can be re-worked until it is better. If an area gets too muddy or confused, scraping is an option--that leaves a ghost image that can then be repainted with more authority.
Try gently laying on those final highlights with a palette knife--much easier to keep from mushing them into wet paint below.