My workflow is:
first planing phase - think of an idea for main mechanic of the game and how to make it. (this is not count as time).
I have to make it for 10, 15 mins max. Then I polish the game and stop at 1 hour. After playtested I take a break.
second phase - it is polish for half hour playtest break and repeat till i hit 3 hours. Break can be as long u need to get back fresh. So overal I make the game for 10 mins then just polish for 2hours and 50 mins . ( polish don't mean add mechanics it's oposite)
My general rule is if the game take longer then 10min to make it's to big of a scope. For 3 hours you can't make a game, but you can make your Core for that time. Often i can't even finish this 10 mins games. I apologies for the broken English. Good luck!