Memtest runs a number of different tests, (or passes) i generally run 4, 5 at most, there have been problems in the past with tests 7 and 8 where it doesn't read correctly
It is only pass or fail. So depending on how much ram you have installed depends on how long it takes to get 1 pass or a potential fail.
As soon as you get 1 pass the test is good. Repeating the tests isn't needed but like most doing ram tests, people run the test and walk away. Then to come back with it having done mulitple passes.
Memtest runs a number of different tests, (or passes) i generally run 4, 5 at most, there have been problems in the past with tests 7 and 8 where it doesn't read correctly