If you dont care if its a little unstable then do it for 1 hr.
If you have the time, and can't afford any instability I say 24hrs. Though an overnight run would do in most cases.
Well I guess that 4.25v wasn't necessary. I've got it at 1.38v just now.
Safe everday gaming = stock speeds Safe gaming w/ slight overclock = 2.5 GHz to 3.0 GHz OC Gaming w/ moderate overclock = 2.5 GHz to 3.5 GHz OC Gaming with a high overclock = 3.5 GHz to 4.0 GHz OC Not entirely safe nor dangerous overclock = 2.5 GHz to 4.0 GHz (this will reduce the lifespan of your cpu though)
It's a 45 nm CPU. Intel's recommendation is for a max of 1.3625 volts.
But what do they know?
I blasted my e5200 with 1.58v at 4ghz and after two 12hrs runs of prime it got somehow "stretched" and now it requires a bit more voltage at lower speeds than it did before i did that firstname.lastname@example.org(it used to do 3.6ghz at 1.32v now it needs 1.36)
ehhe, run the intel burn test, 20 loops and maximum settings. Also watch your temps; they say that burn test will go 20C above prime 95.