Most people run it anywhere from 20 minutes to 24 hours. I'd personally recommend you run it for at least 12 hours. 24 hours would be optimal. Leave it on overnight and check it out every now and then for errors.http://www.tomshardware.com/forum/252077-29-long-prime95
Go to the above link and read what the dude with gold badge says. He's a moderator and I'm sure he's a trustworthy guy.