it all depends on waht u do with ur comp. E.G
i can OC my E8400 to 4ghz and its stable after 7hours orthos but when i play farcry 2 it crashes ( at 3.92ghz its fine)
so really "stable" is what it means to you, if it does what u want it to and theres no heat issues then its stable.
A nice quick test is intel burn test and its free
hope that helps.