I don't have a hard and fast number to go by, but I'd say at least once a month. Don't know if that generalizes though, as I'm a bachelor, and frankly have no squabbles about depositing my ejaculate in the bed every other day. I don't unload directly on the sheets, I'm not that gross, but rather stick my dick into a hole and use the space between sheets and mattress to hide the mess. Still, the fabric tends to somehow 'suck' the cum upwards, and over the weeks it will get bumpy and develop yellow spots. If it starts to noticeably smell and to attract insects (mainly maybugs, idk why), I usually begin thinking more seriously about fetching new sheets from the ol' commode.