How long is a piece of string? 1-infinity hours I guess....
I would say on average I try to keep the total time to around 20 hours, but if it's going beyond that then that usually means something has gone wrong, either in my process or the image itself. The current paintings I am doing are taking a lot less time than previous ones as I am trying to simply things a lot more and make sure everything is almost finalized before touching the final canvas. This has actually made my research, thumbnail and color study time increase by a lot in comparison, however the final painting is taking much time and I don't have to worry about scrapping it and starting again as I have in the past.
The book I just finished had about 3 double spreads that I spent a lot of time on, but completely repainted them at a later stage. It was a waste of time, but it taught me to make sure I was well prepared and I feel that it is paying off as I have managed to paint about 3 paintings in about 3 days as I already had everything set up for it. The painting I got to do for the podcast also felt as though things were running a lot smoother than usual and I was pleased that I got a result I was happy with without spending too much time worrying about it.
This is all through trial and error though and if I hadn't wasted time or gone through a bad process in my pursuit of becoming an illustrator then I may have not learned the things that have helped push me forward