I bought a TA44-5 two years ago... When will the tritium stop to work?? and what I'll have to do?

The tritium in ACOGs is warrantied for 12 years. The half-life of tritium is 12.5 years.

This means that in 12.5 years, using only tritium as a source of illumination (i.e. total darkness, fiber-optic tube on compact gathering no additional light), your reticle will be half as bright as it was brand new (assuming the tritium used in it was relatively new which isn't always the case).

If after the warranty period you want to have the tritium recharged, Trijicon offers that service for $250 for ACOGs.

Hey that is good info.  I love my ACOG and don't think I'll ever get rid of it.  Thought about the tri-power, but I'm hooked on the TA-31F now.  

Thanks Bartholomew_Roberts! Is the service offered by Trijicon only for US customers? last question: Are ACOG water resistant?

You'd have to ask Trijicon about their service.

The NSN ACOG is waterproof to 60'. I don't know how the BAC ACOGs rate on that scale; but mine has been completely submerged in about 6" of water with no issues at all so I imagine they are waterproof as well; but I don't know to what degree.

Worst cased scenario you send it to an "ally" in the US and it gets fixed and returned to you.