As said, Tritium has a half life of 12.3 years.
So after 12.3 years, half the Tritium will have radioactively decayed, the other half continuing to emit beta radiation. After 24.6 years, 1/4 of the tritium will remain.
Will a gunsight be half as bright as new in 12.3 years?
Probably not.
Is the phosphor lining of the tritium lamp that converts beta radiation (flying electrons) into visible light equally sensitive to lower levels of radiation? I don't know, but it is unlikely to improve with time.
Is your eye equally sensitive to lower light levels? No.
So the 10 year "guarantee" by some brands is reasonable.
Now if you want something that lasts, look for some radium paint. The half life of Radium 226 is 1610 years. Wiki says old clocks have quit glowing because the phosphor is burnt out, not because the radium is not still there.
PS: Don't lick your paintbrush to a fine point.