I think that you are assuming that you will lose more than that average. No proof or tests, right?
T12 NO and VHO bulbs have been known to last 24 months, or more, with normal, everyday usage. Unless there is something drastically different with T5s (there isn't really), then this might be a better comparable than assuming otherwise. Horto folks will use T12, T8 and T5 bulbs for three years at 12 hours a day.
I think that for all but the most light demanding stuff, 24 months is probably fine. This is what I recommend to people using MH over softies, LPS and the easier SPS as well. If you are focusing on acros and clams, then that is different and probably should focus on 12 months. If you are overdriving stuff, then probably more like 6 or 8 months.