Smoke unit life span?

Provided you don't burn out the wicking, a smoke unit should run for many years. Some parts, such as wicking and elements may wear out in time, the same is true of the motor, but all of these are easily replaceable at minimal cost. If I were you, I would not lose any sleep worrying about them.

Smoke units are like gearbox grease, the gears don't wear out as a rule, but they require maintenance.

I don't think any one has run the empirical data to know.  Run it once without smoke fluid and you will have some burned wick and a heating element that is over heated.   Does that reduce the life?  Sure, but how much.  Don't know.  I am with Nicole on this one.  Treat it right ad it should last a long time.  G

if it is a recent Lionel unit, any time from 1 hour to years

Two smoke units failures because of negligence, and no failures because reaching a life span.

Two smoke units in my roster of steam engines have required the replacement of their fan motors because over-filling the smoke fluid reservoirs enabled smoke fluid to get into the fan motors.

Here's a tip. When you ship or store a locomotive with a smoke unit in its box, be sure to empty the smoke fluid reservoir.