Meprolight says 'up to 15 years'.
Meprolight, Military Tritium Self Illuminated Sights by Meprolight, Tritium
I read somewhere that tritium sights are half as bright as new after twelve years.
P.S.
Here it where I saw it:
http://en.wikipedia.org/wiki/Tritium
Half the remaining tritium in the sight will decay every twelve years.