6 weeks? That's like.. wearing your clothes for 14 days straight, how would THAT smell. :-X
I think a good rule is once a week to keep them clean. If you wait too long between cleanings, the sheets get stained and don't look real nice anymore.
With six weeks, at eight hours of sleep a night, that's 336 hours you are spending shedding skin and drooling on your sheets before washing them, along with hair, sweat, and oils. This doesn't include any other recreational time reading, doing homework, eating foods, and dirty deeds that are getting done on those sheets, and making your own body dirtier faster after a shower.
Dust mites feed on your skin cells, and the more skin cells in your bed, the more dust mites. Which means more dust mite poop. And more harmful bacteria to eat the skin and poop.