According to the Lighting Research Center:
How long do T5 lamps last before they burn out?
Lamp manufacturers claim that T5 and T5 high output (T5 HO) lamps last 20,000 hours (Philips Lighting 2001/2002; OSRAM SYLVANIA 2002; GE Lighting 2001/2002). This average rated lamp life is measured at temperatures between 15°C (59°F) and 50°C (122°F) when operated on electronic programmed start ballasts on a three-hour switching cycle—3 hours on and 20 minutes off—and designated as the number of hours after which 50% of the lamps fail. The 20,000-hour lamp life of T5 lamps is the same as the lamp life of most T8 lamps, although newly developed prolonged-life T8 lamps have lives of 4,000 or 10,000 hours longer than T5 lamps. Lamps operated on longer burning cycles will have longer life spans. Shorter burning cycles (frequent switching on and off) reduce lamp life. Use of ballasts that do not meet lamp requirements set forth by the lamp manufacturers may also result in reduced lamp life.
http://www.lrc.rpi.edu/programs/nlpi...s/lat5/pc8.asp