I base my need to replace decision on dark shadows developing at the ends of the tubes, but dedicated people replace them every year or after each "run" regardless. New tubes definitely are brighter, so you may want to keep a "test tube" to compare against existing ones.
FIY I bought wrong tubes -- a 10 tube pack of Phillips "Natural Light" (5000K) T-8. I don't know what was going through my head except that I was concentrating on lumens for brightness for some reason and these are marked 2850 lumens
Life: 30,000 hrs
Color Rendering Index: 82
I did become concerned that I was mistaken and also picked up a 2-tube pack of
"Daylight Deluxe" which is marked 6500K and 2750 lumens.
Life: 36,000 hrs
Color Rendering Index: 78
But they also sell "Daylight" which I believe was 6000K.
Standing in front the huge aisle of f. Tubes, I got confused.
After coming home, I reviewed my notes and asked around, and you do definitely want 6500K or higher for starting seeds (blue lights) for good foliage growth. I also mistakenly thought 5000K would be good enough for the "warm" range for blooming/fruiting, but I was told that this color range doesn't do much/as well for plants and they need the 4100K (red lights) -- Actualy called "Cool White".
In addition, last time I was poking around the web, I read that you could put T-8 tubes in T-12 fixtures and still have them work. But last night, I dug a little further and stumbled on an electrical contractor's forum. In a discussion there, consensus was that T-8 tubes/bulbs used with T-12 ballasts shorten the bulb life (and T-12 tubes/bulbs used with T-8 ballasts shorten the ballast life) -- uh oh.
One contractor cleverly remarked that mistakes like this by maintenance crew means more extra work orders for them.
In addition, one person commented that T-12 ballasts last forever (longer than T-8) so one idea wod be to stock up on T-12 tubes before they stop selling them.... But another idea would be to replace the T-12 ballasts with T-8 ballasts which I read once before elsewhere that it is not difficult to do
...then again, T-12's are not as bright as T-8's nor as energy saving. So what to do?
Anyhow, back to the store I go to exchange the T-8's and get some T-12's also. I wonder if you can exchange/return items bought at another Home Depot store at a store closer to my house?
P.S. recycling -- I suppose you could offer them for freecycle/Craig's as used but still usable tubes? Mean they'd be fine for general lighting until they burn out (tube ends turn black and they don't turn on, start flickering or tracering). When they do burn out I take them to my township's hazardous collection site. I did mean to ask if they had a collection box for the tubes at Home Depot (or other big box stores) like they do for CFL bulbs. I'll do that when I go back.
P.P.S. Maybe you could have a sun-loving seed starting section where you would keep replacing the tubes, and shade-loving seed starting section where you can keep using the older tubes.