As long as you like. 🙂 If a system can run this for 72 hours without errata, I'd say your system (at least the memory and cpu) is very very stable. Prime95 TT is very good when doing the initial o/c as it will err out very fast when you're past the practical limit of your cpu. Other applications may seem to run fine unders these conditions, but you can be assured when you reach a speed where the TT bombs out you're near the limit. That's when it's time to start considering higher core voltage or better cooling. (usually both).
Cheers!