Temperature and humidity - despite our best efforts to control them - have an outsized influence on the chemistry of poly resins. Mixing of the A and B....time spent in the mold.... mold temp and time to cure before de-molding has a big influence.

While it is possible to pull parts as complex as a body within hours of a pour, in most cases, it is better to leave the resin in the mold as long as practically possible while the chemical reactions continue to completion. Even tho it is solid, it ain't done.

FWIW, things can happen years hence. I picked up the beautiful '55 Safari from He Who Shall No Longer Be Named in my first trip to Toledo in the mid 90's. Immediately whacked the hood to add detail (not sure what the end project was going to be) and fitted up some leftover TriFive stuff and was at a decent place when it went back on the shelf. Perioically checked in the ensuing years... then, just before the move, opened the box to do a quick inventory and the hood looked like a potato chip while the rest of the body was fine.

Again, one body and one builder's experience. I have ancient Modelhaus that is just fine, for example. Other name caster items had issuses. Best bet: build the darn thing, and move on!