How Well Does Ball Tritium Tube Lume Hold Up? Real World Experience?

Tritium has a half life of 12.5 yrs +/-. Ball claims their tritium tubes have a useful lifetime of 25yrs. That's 2 half lives, so by 25 yrs they should only glow at 1/4 of their original intensity. Or do the tubes originally contain enough tritium to more than saturate the phosphor, so that even after 10-15 years of tritium decay they still glow at full intensity?

What is the real world experience? I'm new to Ball and have one 7 year old watch that is impressively bright, but wonder how it would compare to a new watch. If you have more than one Ball watch, how does the intensity of the tritium tubes in your newest watch compare to the intensity of your oldest watch? What's the age difference between them?

Thanks for responding.