How long i need to stress my pc with prime95? 30 minutes? 1 hour?
As long as you can. Set it to run before you go to bed, then let it run all night. When you wake up, check to see if it's still running, if not, you need to change something. If it is running, great. Let it keep running while you go to work or school (whichever applies), then recheck it when you get back. If it's still running, you can probably call that stable. But no, 1 hour is not enough.
But keep in mind, it might be stable with a stress program for days on end, but the OC might still cause app crashes and glitches. The only true way to test for stability is to use it like you normally would, and see if anything is unstable. Those stability testing programs are only to give you a general idea.
I know that's a fairly vague answer, but unfortunately, that's as specific as it can get. There are no set amounts.