To run a proper Memory Test of any kind,, weather it is MemTest or Windows Memory Diags
You should let the test run for no less than 4 to 6 hours solid. 5 Passes is ok for a quick test, but if you want to know that all of your RAM is good, then it should be run no less than 4 to 6 hours on 4G or less of ram. If you have more it should be generally exponential to something like,,, 6G no less than 6 to 8 hours,,, 8G+ no less than 8 and up to about 16 hours depending.
One thing to keep in mind is,, you can run Memory Tests for too long where they will begin to generate errors even on good solid ram. we are talking approx. +/- 24 hours.
But less than 4 hours on 4G or less is not long enough to determine that ram is as 100% solid as can be determined by a ram test. Ram does go bad, but if you do not get any errors within that 4 to 6 hour time frame, you can be relativley Highly Certain that the ram is good and will last a long time.
Unless, you get errors right away. Any errors right away and you can be pretty sure you have a bad stick of ram or bad slots.