Supposed to last for 2 years, that was one of the main reasons I switched over to T-5's. Mines been up and running since January with considerable growth on my SPS's when compared to the 110W VHO's I replaced.
I will probably change them at 18 months. The 6.5K GE Starcoat F54W/865 HO bulbs only cost $7.49 each. Biggest expense will be the T-5 blue bulb that I use to even out the yellow color, about $25.