I let it run two times for each module, then both modules, if all ok, then swap slots. Usually, for me, if it's really a faulty module, it starts popping errors already during the first pass. Otherwise it's usually something else at fault.
I think it really depends on what tasks the computer is going to perform and when it is needed. At my last employer, I had a spare lab PC that run MemTest continuously unless it was needed. It once ran MemTest for several weeks, and some random errors were reported.