Stupid question about t5's

[sharuq1]

I know this is a really dumb question, but my brain isn't working properly (that's my story and I'm sticking to it) :

How long can you run your t5 bulbs (normal usage for a reef tank, in say a Tek fixture-stock not overdriven to heck and back) before you need to replace them? I have heard everything from 6 months to two years and this seems like the best place to get that info.

Thanks all.