The epoxy in the box is more for fuel proofing a patched hole than anything else. It will cure sufficiently for that in 10 min.

I have seen fuselages snapped in half, repaired with 5 min epoxy (and some sticks to splint it) and the model fly within 30 min with no problems at all. It really depends on HOW it broke and how you do the repair.

Generally, if a model is repairable at all... you'll find someone who wold be willng to do that level of work at the field. That doesn't mean its safe or unsafe... Some people are better at field repair than others. Some have more supplies and tools at the field than others.

There is a segment in one of the TOP GUN videos (I think 2000... but not sure) where a contestant rebuilds a severely damaged wing, at the field, then proceeds to fly the plane again at the contest. (and it flies just fine.)