I base it on miles: I wear mine for about 25 miles or 2 days of commuting. I figure if I was going on a long ride I'd wear those shorts for 25, 50 100 miles....it's not going to hurt me to wear them 2 days in a row
oh: and I don't use chamois creme - even for my long rides....never have.
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"