The mean time to failure (MTTF) varies slightly depending on the series.  See the below list and attached supporting documentation. 

M100 - 28 years

G100 - 27 years

C100 - 23.6 years

S100 - 22 years

H100 - 22.7 years (9 years for 110kW~500kW)

iS7 - 19 years