If you’re talking about
memtest86+, as in the bootable programs, sure. Interrupting it won’t do anything, since it never writes any persistent data. In fact, the tests are intentionally endless – it’ll just keep running passes until you stop it.
memtest is structured as a number of tests, each of a specific pattern. A single completed run through all selected tests is known as a pass. As mentioned before, you can safely abort at any time, simply by switching off the machine.
There is no optimal number of passes – an obvious failure will be caught in the first pass, while intermittent failures might take a hundred passes to appear (at which point you might as well get ECC RAM). I used to recommend running at least 10 passes to catch the more common intermittent failures, though with larger RAM capacities these days it could take too long.