Tritium has a half-life of 12.3 years. So, for that reason alone, the illumination will be half as bright in 12 years. Plus there is some degradation of the phosphorus, so the decrease in brightness is faster than half / 12 years.
But the human eye doesn't perceive half as many photons to be half as bright. Our eyes can see in such a wide range of light conditions, that that a source that seemed bright enough at t=0 might still seem bright enough 24 years later, even though the number of photons emitted is 1/4 as many by that point. That was my experience with a tritium back-lit watch (it is an LCD digital watch, with the entire LCD display backlit, not just the hands and 12 point on the dial) from 1978 - now, 10 years later, it was still bright enough to see clearly in all light conditions (by reflected ambient light in bright conditions, by tritium/phosphorus light in dark conditions and a combination in dim light). But now, 40 years (yeah, I still have it), it is quite dim and I have to be in a dark closet with my eyes fully adjusted to see the glow. So it is not a practical backlit anymore.