I find that odd the tritium already needs recharging I thought the half life was 12 years , but I will admit I could be wrong. I have a ta31f I bought used 10 years ago and it still does well . either way for 188.00 to get recharged and a new fiber optic and tube thats a pretty good deal. they are an amazing optic congrats on your purchase.
It would vary depending on how bright the tritium cells were when new, as well as the individual's night vision. (Old eyes?)
While the half life of tritium is fixed, if the reticle is just bright enough when new it won't necessarily be 14.5 years or whatever before it seems too dim.
I bought one of the very first ACOGs made, serial number 2XX. Very shortly after buying it I found myself in the deserts of northern Saudi Arabia for Desert Shield/Storm. When new, the tritium in the ACOG was in my opinion actually too bright. At night in the desert with fully-night adapted eyes, the reticle was so bright that it actually washed out targets. That's a tradeoff, the brighter cells will last longer before they become too dim to use. IMO after about 5 years the tritium illumination was perfect. Now, after 27 years, the reticle can only just barely be seen in total darkness.
I have been thinking about sending that old scope in for new tritium, so I appreciate the updated info on cost. As I understand it I would also have to get a new reticle with the tritium cells, which is too bad because I have the old skool 1st generation reticle that they don't make anymore. I'd probably change it to the USMC TA31RCOM4 carbine reticle.
EDIT: The daddy of the ACOG was the British L2A1/L2A2 SUIT (Sight Unit, Infantry, Trilux). Very similar design, but the Brits put the tritium ("trilux" in British) in an adjustable knob, so by turning the knob you can move the tritium cell closer or farther from the reticle to control the brightness. I wish the ACOG had retained that feature. Plus, the knob comes off with just a circlip and the tritium cell can be replaced pretty easily right on the kitchen table.