It took about 14 years for the lenses on my Expedition to get cloudy, and that was parked outdoors. the lenses on all my smarts combined have not even gotten near as bad as what I have seen on other cars. Depends on location and circumstances I would guess.
But then I have not seen any peeling clearcoat on my smarts either, just a glazed roof on my latest smart.
Luckily no signs on my smart headlights. On some previous vehicles we had a hard time with it (i.e. 2003 PT Cruiser, 1993 and 1996 Jeep Grand Cherokee's) to the point of polishing the lights every 8-12 months to clear them up.
Prying the front ens off isn't that tough to do but I have not fond anyone who sells a replacement lens yet. Replacing the whole assembly is not cheap. I have used a few different "restore' kits and they work pretty good, but they seem to be more susceptible to discoloration afterwards unless you coat them with some sort of UV protect ant.
It appears the fortwo's headlights (at least the 451's) are of a more hardy polycarbonate than most other headlamp housings are. They should last a very long time before hazing...but make no mistake, they will haze at some point. There are some 2008s in my area that while they aren't yet cloudy, the housings are definitely beginning to turn yellow.
Likewise, our cars should last an exceptionally long time before rusting too. Assuming somewhat regular washing if you live in a salty area, of course.
The repair kits are all kind of a temporary patch kind of thing. Your best treatment is to sand the lens clean with finer and finer paper until it is smooth and uniform, then use a high speed buffing wheel with finer and finer compounds until it's clear again. Then topcoat with a high-quality automotive clear. You can't get rid of the yellowing unless you get rid of the oxidized layer of plastic on the surface.
the restoration kits I have used were all using a multitude of buffing pastes to polish the lens after removing any scratches. That removes the top layer of plastic to clear the lens but did not include any sort of final coating in the kit.