Depends on what you define as damage. Are you talking about hits, or actual parts missing or damaged?
for hits, the Luftwaffe estimated it took an average of 17 hits to destroy a B-17 (from a 190, don't know what model, or what gun configuration, guessing the 30mm cannon in an A8)
For actual damage, it can keep flying as long as hit has 2 engines, wings, tail, and nessicary control surfaces, past that, you can rip a hole 3 ft wide and it'll keep flying.
In the game, its untill you destroy a criticle component, or untill it just up and explodes.