Well, it depends on whether 80c is your peak temperature. On my i7-920 I find that prime95 peaks about 20 or 30 minutes in. 90c is my cutoff point. I don't even let benches run past that point. But to answer your question, you can leave your system running @80c for 10 hours to fully test stability if you like. If you haven't run IntelBurnTest, it's quick and often discovers instability. Then if temps are still good run Prime95 for about 8 hours (but it's good to occasionally check temps during this time). But generally prime95 will hit peak temp within an hour or so.
Yep - I've found that using the broadest range of stress-tests works best. Also remember that stress-testing is indeed stressing your OCed cpu, so don't go too crazy with it over too long a period of time. No need to wear it too much : ) Anyway, have fun!