In terms of a time limit, the powers would remain until the threat is no more. In the scenario where a tank falls on top of you and you'd develop super strength or toughened skin, I'd expect the abilities to last until you aren't under the tank and being crushed anymore. Any sooner and you'd probably be dead.

As for other limitations, I think having it only activate in the presence of an incoming threat and only being temporary is a pretty good weakness. When not faced with anything, my character would just be a normal person. 

Comparing this to the other abilities and weaknesses of characters that have already been accepted, I think this seems pretty fair. The abilities Diego copies seem like they're permanent while only having physical contact as a prerequisite as its weakness; not sure if I missed something though but that doesn't sound like that much of a weakness. Also, if I sound rude in anyway, that wasn't my intention. I just wanted to point it out as an example and this was the best I could find.