Believe me, it goes from being hyperfocused (that's what it's called), to being bored out of your skull.
But the challenge will keep you going for a while.
Once you put brush to door, 30 min. is too long. I spend time taking off all the hardware except hinges ( on a repaint). Then I tap a little bit of paint in all the corners around the panels and cut in the hinges. Then I use a mini roller. With the corners colored already, a mini can be used on flat surfaces w/ a brush handy to put excess paint out of corners if necessary. Usually the roller won't fill the corners.
If the doors have a grain, then I roll with one hand and brush it out with the other. Very quick method.
Thin coats.