Most people say at least 8 hours.. I personally leave it running overnight...
Though I prefer to use LinX now to check for stability, it is a lot quicker, though once I get my system stable with 20 passes of LinX, I then check with Prime95 Blend Test overnight just to make sure my system is stable.. if it passes both of these tests and a few loops of 3d Mark Vantage I class that as stable...
Everyone is different though, some people only run Prime for 1 hour and if it doesn't fail any threads in that time they class it as stable, though I have seen P95 go for several hours before it BSOD's... that's why I prefer LinX...