I believe that the tests just keep repeating. Sometimes the memory problem is intermittant. I would think that 7 - 10 hours would be adequate.
BTW what is it that makes you believe it is memory problem?
If you have more than one memory module installed you might consider pulling all but one and running them each individually to see if that turns up a problem.
MemTest is an excellent utility. Another memory tester that is free and you may want to try is DocMemory:
http://www.simmtester.com/PAGE/products/doc/docinfo.asp