The most accurate answer is that--given solid sanitation, of course-- you won't ever be harmed by leaving it for four weeks. Whether there's significant benefit from the extra two weeks is a matter of some debate here.
FWIW, my everyday beers (1.070 or less) almost never go beyond 10 days in fermenter unless I'm too busy to bottle or keg them, but then again I pitch the right amount of yeast with a starter and closely control my fermentation temps. I believe the current HBT party line is "4 weeks minimum", however, with an eye to letting the yeast clean up after themselves. As I said, there's generally no harm in it!