Quote from memtest site:
"How long to test: Executive summary: 100% coverage represents a very thorough testing your memory, and will catch all but the most intermittent problems. To catch those much more rare intermittent errors run to 400%.
Empirically we have found that the vast majority of errors are found in just a few minutes. 60% of bad RAM is detected after just 10% MemTest coverage. Running the test 10 times longer (100% coverage) raises the bad RAM detection rate to 95%. The remaining 5% are intermittent errors. Unfortunately, there's no way to speed up detection of intermittent errors - you have to wait for them to happen. The same pattern may be stored accurately the first 1000 times it is written, only to fail on the 1001th write. We have found that testing to 400% coverage will catch almost all intermittent errors, but rather than trying to hit that number exactly, we recommend testing overnight. Your computer is not doing anything else at night anyway. Or, you can run MemTest Pro while you use your computer for other tasks, which can also help identify memory errors which only show up while the computer is under normal load.
MemTest will report any errors that it finds as soon as it finds them, so if you do not see any error messages then all testing so far has been successful. Once you start testing MemTest will continue to test your RAM until you tell it to stop, or quit."