The official halflife of the molecule hydrogen-3 aka tritium is 12.32 years. Which means in 12.32 years it will be half as bright as it was brand new. In 12.64 years it will be a quarter as bright as brand new and so on and so forth.
But that's not to say how effective it is as a light source.
Assumption is the mother of all ****ups.
Know what you know, Know what you don't know. -Paul Pedzolt
"Jack of all trades, master of none, though oftentimes better than master of one" -Mynshul