The life of a T3 head, or any wick/atomizer assembly for that matter, will depend on how fast it gunks up, IE accumulates black carbon based residue that insulates the coil eventually to the point where vapor output is very low or stops altogether. Sweeter liquids, fruit, candy and confection flavors all tend to produce the most gunk since the sugars in them contains lots of carbon molecules.
Rinsing them when refilling will help extend their life a little, but that gunk is kind of like concrete. You can also "dry burn" them, carefully, to remove most of the gunk and replace the flavor wicks, loose wicks, to extend their life quite a bit.
Just my opinion, worth what you're paying for it. Your results and mileage may vary. No warranty or guarantee is expressed or implied. Proceed at your own risk.
Thanks awsum for the awesome info!