You can't count on 2 or 3 half-lives before the luminosity decreases beyond limits of detection...it really depends on how much tritium was encapsulated to begin with, as well as HOW OLD THE TRITIUM WAS when they made the capsules.
Its my understanding that there has been no tritium produced in this country for a number of years, since they closed the Savannah River production reactor, I believe in 1988. The military obviously has first dibs on the supply, and tritium is one of the most expensive commercial commodities around. Recycling from warheads probably accounts for the majority of the supply, and possible some imports. Hence the expense and lack of purity. Some of the capsules, if they contained refined tritium and not the He3 decay product when manufactured, may have been sitting around for a good while before incorporated into the end product.
So.....you never know what you are going to get, and how long it will last.