Welcome Jessica!!! This is a great club with lots of knowledge and willing to share it.
Jessica, Dave is absolutely correct on his statement but I disagree with the last sentence (I'm not saying he is wrong but I believe to be otherwise, however that is subject to individual interpretation).
I have read that if the T5's are not overdriven the can go between 21-24 months on the good side of the spectrum, now general consent by most people I know (myself included) will replace their bulbs between 12-18 months.
Raf