I'm willing to bet it's pretty far off the norm, I'm sure there are people who do but even the most cleanliness obsessed person I know isn't changing them that often.

Sweating that much in your sleep is not as common as you think.

I've met few people who do sweat so much if they are heavy drinkers at night though. I have an ex that started drinking heavy when his best friend died and you could see the sweat stains in the morning. Never used to be like that before he started drinking. Thankfully he was raised to do his own laundry like any capable adult should. Didn't save the mattress though.

Even medically with dust allergies they perscribe to change once a week. Weirdly they print it out too to hand it to you like it's a guide.