I dont' have a hard and fast rule, but I've often acquired the minis for an entire platoon, army, etc and then decided to let them all go. If the pile has been sitting there and there are several other large projects (also likely stalled) that I like better, I'll let them all go. Luckily most of these projects come as a result of a super-deal bargain purchases so I can always break-even and often turn a bit of a profit, even at the clear-out prices I tend to charge.
I've done this for.
-Adeptus Arbites army (arbites, Junkers, resin vehicles etc) though I did start a Junkers army years later...
-Kryomek Cyclos force
-Ork Mob (50 figs or so)
-Warzone platoons
-Huge Medieval/Fantasy human force 150+ figs
-Massive 3 Faction Spacelords project, Much original 80's MISB...
and others...
It almost never really feels like a loss (except for the Arbites, wish I had them). I look at it as a privilege to be able to acquire the figs, see their sculpting/artistry first hand, increase my knowledge of miniatures and then send them on to someone who will enjoy them while I roll the money into something different.