Those are all good questions.
I'm sure that trijicon date codes their scopes. Though you'd probably have to ask them to date it if it isn't clearly written on the scope. I know that with most all night sights, they do have a year date printed because of their warranty.
With that said, tritium is a radioactive isotope. That's why it glows. It doesn't necessarily wear out per say, but it does have a half life where it gets weaker and dimmer over time, until it no longer glows at all. Or, if the vial is damaged and the gas leaks out into the atmosphere.
I have had trijicon replace the night sights on my Beretta (the front sight is fixed and the user has no way of changing them on their own). They did with a minimal charge to replace the vials in the iron sights. I believe that it costed about $65 or so. But it may not even cost that much for an acog. Again, you'd have to contact triji and ask them if/when you need to.
Just a note about my night sights on the beretta, 2 of the 3 dots were still glowing faintly even after more than 20 years. I think their warranty is for 12 years ??? You'd have to look it up online to be sure because I'm just going off of my head what I think I remember reading.