This is always an issue with body repair and one with no absolutely clear answer. There are cars with two inches of filler that go for years with no problems, and then there are cars with a paper-thin layer that lifts off within months. It seems, therefore, that the "secret" of filler's success or failure is the underlying surface.

The technical data seems to indicate that a 1/8th inch layer is perfectly safe, and building up layers to 1/4 inch shouldn't cause failure later on. This assumes that the underlying metal is stable, solid and rust-free. Cracks and holes must be welded because any movement in the metal will crack the filler on top. If your car is of unibody construction, bear in mind that the whole thing twists as it travels along the road. Make sure the metal structure is absolutely correct and you shouldn't have any difficulties.