I use them until they die or start tasting really harsh/weird. I've noticed that just before the coils either die or pop, they usually give off a really harsh, metallic taste. That's generally when I tend to swap out. At first, I tried changing wicks when that happened, but it proved completely futile. The wicks I changed weren't even burnt. I've gotten a week out of one that had become like that, but it wasn't pleasant. Shortly after the taste reached its worst point, the coil blew out.
Generally, I get 4-5 weeks out of them before they reach this point.
I vape menthol exclusively, so I tend to have just two "up and running" at a time. I alternate when I clean, which I do once every 5 days or so by removing the wicks, dry-burning, and rinsing under warm water.
I've often wondered if grain alcohol somehow shortens the lifespan of the coils. When I first started using T3's, I was also using vodka (I've also used everclear) as the base for my menthol solution. My coils only lasted two weeks at the most when I was using juice with that solution mixed in. Now I use PG for my menthol base and my heads last twice as long. Granted, I've only been using T3's for a few months now, so there could be other factors at play.