Max build time for a unit (and presumably a building therefore) seems to be 244 turns. I say "seems", because I haven't actually gone through 255 turns to see if it's the case. I'd set a unit to a build time of 800 turns just to make them effectively unbuildable, but at least retrainable. Anyway, in game it displays their build time as 244 turns. Fairly trivial really, but there you go.