Trijicon says theirs will last 12 years, I assume from date of manufacture. Cheaper/lower quality may last less time... and abuse may could probably shorten that life span if it somehow compromised the glass cylinder that holds the gas. They're around $100 or so, maybe less if you can shop or get an inside deal.
Okay, they are not "dead" but mostly dead. I put the gun away in the safe and in the almost total lack of light I could see some green. I think HiViz is what I will be going with next.