Wow. Corrections are in order here.
1) There is no hard rule of when bulbs should be replaced. It can be anywhere from 6-18 months depending on bulb, ballast used, properly cooled, etc. BRS did a full blown test to prove the good bulbs last 14-18 months when properly setup. They have PAR and spectrum data in those videos.
2) If this is for a fish only, run them til they die. Nothing in the tank is relying on PAR or spectrum to survive.
3) ATI, Giesemann, and KZ are great bulbs. The same manufacturer makes both ATI and KZ bulbs. It’s either Philips or Sylvania. I don’t recall. There are only a couple manufacturers still building T5s and they are all massive corporations that make them for hobby companies.