About Contactor Lifespan
MIL-HDBK-217 provides a calculation to help “fine tune” a reliability prediction as it applies to a specific application.     

It consists of the following formula:

\$λ_p = λ_b · π_L  ·π_C ·π_{CYC} ·π_F ·π_Q ·π_E \$ = Failures/\$10^6\$ hours.    

MTBF = \$10^6/λ_p\$ hrs

Whereas:

\$λ_b\$ = Base Failure Rate (temperature factor)
\$π_L\$ = Load stress factor (load level and type)
\$π_C\$ = Contact form factor (DPST, DPDT etc)
\$π_{CYC}\$ = Cycling factor (cycle rate)
\$π_F\$ = Application and construction factor (general relay load rating and armature type)
\$π_Q\$ = Quality factor (mil-spec qualification level vs. non mil)
\$π_E\$ = Environment factor (environment in which the relay is being used)    

Below is an OMRON life expectancy on relay contacts that shows the effects of inductive loads.  Mechanical Ratings in million operations are no load ratings.

Ref  MTBF Telednye
Crydon


  When inductive Arcs cause contact temp rise, MTBF drops quickly.  This is why Tungsten start surge , inductive stop surge and DC inductive has the lowest rating life.  Resistive loads have no stored energy to dump unless tungsten lamps , which have 10x peak cold surge curernts.