tl;dr:(1) There are enough "known knowns" in the project i.e. things you're aware of and understand. (2) There are no more "known unknowns" i.e. things you're aware of but don't understand. (3) There are too many "known unknowns" - this might be a good indication that you need more info, and a prototype can focus your efforts. (4) You stop learning new things. (5) Falling down the rabbit hole.