"How much longer can I resist" is the correct translation given, however, "how much more can I withstand" seems a closer fit.
I don't think so. It literally means 'how much more'. You could also say ne kadar daha.
I think the fact that we don't translate the word 'longer' confused you. Yes you could say Daha ne kadar süre dayanabilirim to sound more clear but you don't have to. Ne kadar can mean how much time and how much money by itself.