Whenever they start to make noise. You'll notice they're worn if they squeak when wet or if they're noticeably cracked.

I replaced my alternator/AC belt at 150k miles and it looked fine when I took it off. Easy job to do yourself. I dunno what the F series engines are like, but my J30A1's PS belt wraps around an engine mount and behind the other belt so it's quite a bit of work. It's been making noise when wet so I need to change that one h: