I've run malwarebytes, now how often do I need to rescan?

Sorry to keep banging on about this but the full scan found 17 infected out of approx 70,000 files etc. None were said to be 'malicious' (which is presumably why Norton didn't pick them up?). I've apparently deleted them. So now how often should I rescan, and do I need to do a full one as it took forever!