I can't answer specificly about the fan on the Hot Blast, but if an electric motor is kept lubricated and clean of dust accumulation, it should last many years..
The big issue I think is the dust/cobwebs/flyash environment that these motors and fan assemblies operatate in... The dust etc coats the fan, coats the cooling holes and passageways, then the electric motor starts to overheat... Heat softens, then destroys the thin insulation on the wire windings in the motor, then.... mushroom cloud and no more motor.
I recommend a pass with a vacuum every week or so to keep the cooling vents clear, and maybe every month or two, shut it down and clean the fan blades with a q-tip or similar device. Or what ever works for your motor and fan.
I have fan motors that are over ten years old, workng fine,, I treat them as I describe above.