I used T3's for quite a while, and still have a couple, though I use come complicated crap lately...
You can tell when an atomizer needs replacing (or cleaning at least) when the quality and quantity of vapor begins to degrade.
You'll get a "feel for it" - but basically it's "weaker" when it's getting too gunked up to work properly.
They will occasionally literally burn out - and just stop working - then you toss it, or learn to open them up and rebuild (that's fiddling with the tiny wires and wick inside the "coil" or "head". Not necessary, but possible.
I used to rotate a BUNCH of clearomizers (almost all Kanger) - and I'd take the coil heads out every few tanks, run them under hot water, blow through them a little, then set it aside to dry - for a couple DAYS - cuz I had extras in rotation, and wet wick doesn't vape well AT ALL.
This process kept my coils working for weeks - hard to gauge exactly due to having many in rotation - but a pretty long time!
SOme people do a similar process, but take cleaning to another level - soaking in Vodka works to get more gunk off...
Good luck, and good vaping!!!