In my opinion, it all depends on your role-model that you’re trying to emulate. For example, Alvaro Castagnet might finish his watercolor piece just like that in 2 hours and it still looks gorgeous.
About doing the drawing over again, absolutely! I was just looking at all the timelapses of my works till date and I realized that anytime I razed whole or majority of a drawing and did it over again, those drawings turned out to be some of my best drawings.
Lastly, I know I’ve learned from a piece when I’ve made one of my biggest mistakes ever in it.
I just composed a few points I learned from critiquing my own artworks this week. I am thinking of posting the list in the forums later today.