For general stability 2 hours of prime is sufficient, but that isn't going to guarantee you a crash free system.
If you want to be closer to 100% stable I would say 8-12hrs prime as well as at least a couple hours with some different stress tests such as intel burn test, OCCT, LinX, etc. Plus you need to do some real would tests i.e. gaming, multitasking, watching videos. If you experience no crashes during all of that you can be very competent that your overclock is stable.