I was just playing with a torch and my Seiko SKX007 while waiting for my son (dad's taxi) and wondered if I keep doing this again and again and again if it will eventually get less luminous. Or if it's just after 10 or 20 years that it fades.
Tritium lume will degrade over time. My 7548s and 7C43s no longer glow very well. However, Seiko's modern proprietary lume, i.e. Lumibrite is strontium aluminate, which is non-radioactive. Therefore, it is hard to see how this would degrade, unless it is chemically unstable. As far as I can tell, strontium aluminate is pretty stable chemically, thus it should remain highly luminous, when excited by light, for many years.