Absolute fact.
Gross.
I typically only wear underwear in my business casual clothes. I used to not wear them, but people were making me uncomfortable as I'd catch them looking at my crotch (I swear this is true). Oh, and I wear underwear when golfing because it gets hot out there.
That being said, I do use some clothing multiple times:
T-shirt around the house - 2 days unless I get sweaty or something.
PJ pants/shorts for around the house - a couple of days.
Towels - 3 uses (scientifically proven to be the correct amount...seriously.)
Jeans - A couple times.
Everything else once.