I change out at the one year mark, mostly cause I had noticed a bit of color shift of the Aquablue special bulbs I used. The ballast used to drive the T5 would make a difference too, since ballasts like the Icecaps overdrive the T5.
I use the aquablue specials, actinic +, and the UVL super actinic T5's. The T5's as supplements to my MH's and for dusk/dawn effects.
HTHs