umm, i sleep between sheets on a random basis ( i mostly just drop off wherever i happen to be, most often the sofa ) and i change them when i think "hmm. it's awhile since i changed them" or else when expecting guests (whichever comes soonest)
i would need to note sheet-changes in a diary to be able to figure out how often that comes to. and i would also ned to figure out a walkie-proof method of keeping my diary in a place where i can find it more often than not ; both of which would be highly novel circumstances.
you're tallking to somebody who got to the menpause before she got the hang of having monthly periods. they always arrived unexpectedly, cause i'd forget when the last one occured long before the monyth was up. amnd yeah, that sometimes occasioned a sheet-change, not to mention a good long soak of the sheets before washing them.
the feel of freshly-washed sheets next to skin is heavenly, but it doesn't last. the next night, the sheets feel unwashed again (by comparison) , and the following night, and the night after that. if sheets that really need changing feel or look any different , i've never noticed that difference. hmm, so maybe i've just happened to change them in time, every time? :shrug: