I do Bikram, and I always seem to have people around me, usually other women, who have the smelliest, nastiest mats you could possibly imagine.
I toss a mat after about two weeks, if I'm doing my normal schedule. The cleaner deodorizer stuff is mostly useless, and I want to avoid chemicals anyway, so it's easier to just start with a new mat a couple times a month.
I don't think some people ever get new mats. It's gross.