It depends on a lot of things. My builds usually start out with a relatively simple plan, but morph into complex problem children over time, as I see ways to improve on the original idea, or add just one more little mod, etc.
Early in any build, I'll typically spend more time in each session, getting the major work done so I can sit back and look at a mockup that's fairly well representative of the overall concept.
As things progress, I'll set small, bite-sized session goals, so when I find I have a few minutes and am in the building mood, I can accomplish something tangible even if I don't have an extended time window.