I'd say 200,000+mi
The Envoy has 130,000mi and it's still kicking like new.
It seems that the smart could last awhile.
At Six Flags: Great America (my 1st job

), they have a Plymouth Voyager (painted to look like the Mystery Machine) that has 300,000+mi on it and it sounds like it's fairly new.
So comparing to other cars, I expect a smart to be equal to its peers or even better.
So I'd probably drive mine (when I get one) until I'm unable to drive anymore.
Then I'd just pass it down to someone else in my family so they can drive it til' they can't drive anymore...