I always let Memtest run at least overnight, and sometimes 24 hours. Remember that some memory errors only turn up as the chip(s) heat up. I actually encountered a bad batch of RAM chips (several!) with one build which only turned up after several passes (then more than 1 board tested bad). Got them exchanged and new ones were fine (phew!, I thought I had done something to temp regulation to fry them).
Take the time to get memory tested fully.