every few months, depending on how the weather's been and how much i've been riding as well. Definitely replace is after what i feel is the last snow. Sometimes when I'm doing a full cleaning of my drive train I'll be soo disgusted by the chain I'll just replace it rather then try and get the grit out.

Probably about 4 times a year I replace it. $60 a year to insure the bike's running properly is fine by me.