Most night sights can be re-lamped at the factory that made them (Trijicon being the best at this). You just send in your slide, they remove the old viles and put in new. I had the night sights on a 3rd Gen S&W re-lamped for around $50 a few years ago. It was about half the price of buying new.
In my experience they are at full brightness for around 7-8 years, then they slowly dim for another 2-4 years until they reach the point that they are not really serviceable as night sights anymore. The half life of tritium is supposed to be 12 years.