Generally I don't replace media until it's completely falling apart. as long as it's rinseable, it's reusable IMO
It's important to note, though, to only rinse using old tank water. Rinsing it in "like-warm running water" will kill the bacteria (unless you don't have city water or a treated source). So typically when doing a PWC
you'd want to collect a few gallons of water in a bucket and use that to rinse with. Contact with any chlorine will almost instantly kill the bacteria.
Also, do keep it out of the water any longer than necessary when rinsing, as this can kill the bacteria as well.