I have a few and they are very well taken care of. Still curious to know, what is the average lifespan of L lenses ( and non L lenses as well, USM version )
I have an L lens that is 30 years old and still works fine. It's a fully manual lens however. Basically the glass and metal will not age, the only concern is the electronics. Whilst ICs don't really wear out, capacitors do age, I believe caps failing is one of the common causes for old electronics to stop working.
I recently dug out my 1989 Nintendo gameboy, which is about the same age as the oldest of Canon's electronic focus lenses. I'm pleased to report I powered it on and it still works.
Personally I would expect 15-20 years, quite possibly longer. My oldest USM L lens is six years old but I'm not concerned about my lenses ageing, the ones to be wary about would be lenses like the 85 f/1.2L and 200 f/1.8L which have focus by wire and are therefore useless without the electronics.
If well maintained, I'd say 10+ years easy.
Lenses tend to become outdated before they actually wear out, and this is one of the reasons that people often recommend focusing on good lenses over a good body.
Also, mountings change, but despite that, there are still many used lenses that are still used on new bodies with adapters. The abundance of adapters for old lens mounts should be a good indication of the long lifespan of a good lens.
Zooms wear a lot faster than fixe-focal length lenses (though internal focusing/floating element fixed focal length designs are more like zooms).
Most of the lenses I use regularly are between 20 and 30 years old, and work just as well as when I bought them. One is probably due (or maybe even overdue) to be cleaned and lubed, but most of the others don't even seem to need that yet. I'd guess they'll still be fine well after I'm dead and gone.
I would guess the average lifetime of handheld camera lenses is affected mostly by accidental damage that renders them unusable and beyond economic repair.
For lenses that haven't been dropped, crushed burned or drowned, the average lifetime (in which they can be used to take photographs) is certainly measured in decades.
I just tried a 1982 lens† on a 2012 camera‡ and it works as well as the day I bought it.
For lenses that are protected from mould and not heavily used (e.g. by busy professionals), the average usable lifetime may be centuries (assuming you can find a working body that will mate with the lens)
Build quality must be a factor too. I wonder whether current plastic lenses will mostly fall apart with use, long before much older metal-bodied lenses do.
Camera Bodies don't last as well
Cameras on the other hand are often full of parts that deteriorate whether in use or not (leather bellows, plastic sponge mirror dampeners, etc) as well as moving parts which wear significantly with use (e.g. mechanical shutters) or rely on consumable parts that become unobtainable (e.g. OM-1 batteries?)
† Micro-Nikkor 55mm 1:2.8 (manual-focus AI F-mount for 864mm² "sensor")
Manual lenses seem to last forever, well stored and if mold doesn't settle in anyways. Electronics tend to die sooner because capacitors break down over time.
I have a perfectly good 5cm lens that came with my grandfather's Leica M3 from the mid 1950's and I've used far older 4x5 lenses set into modern Compur shutters before. So that's a range of glass that's 57 to 70 years.
Then I have a couple Nikon lenses with electronics in them that are 15+ years old now and still going strong, though a bit worn in mechanically.
I tend to treat my lenses well, clean the electronic contacts, store in normal humidity, etc... I've only had one lens get mold and it was terribly abused camping in the snow, big temperature and humidity changes, all that.