when to replace t5 bulbs? How long do they last?

Is it time to replace my t5 bulbs? How long do they last before growth starts to slow? Ive had mine going for about 15 months.

Thanks

change them. they are good for about 8 months then start to depreciate from there. has to do with the glass becoming polarized. dont know much about it i know most companys will tell you every 6 months but i would say every 8-12 months depending on the bulbs design and manufacturer.