Tritium's half life is 12 years, as mentioned above. Which means in 12 years it will be half as bright as it is now. Then is another 12 years it will be 25% as bright as it was new. So in 12 years I bet someone will have figured out a better sight, or maybe it is just time to buy a new PS90. I bet eventually Trijicon or another company will find out a way to replace the tritium lamp.i seriously doubt you can replace it on the stock stight. but the half life of tritium is a little over 12 years. that'll last long enough for the next gen of battery-less sight that is sure to pop up in the next 50 years.
/1 edit for grammar
Is the trituim in the black ring sight replaceable or does the sight become useless when the trituim wears out?