According to this question, proper maintenance & treatment is need to ensure a long lens life.
But other than occasional cleaning, I've never done anything to any of my lenses.
What else should I be doing to ensure my lenses outlive me?
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"Pay attention" is the big one. If your lens starts to develop a significant change in feel or noise, it's almost certainly due for a service. Don't let it get wet, and ensure that if you store it for a while it's not too humid. If your lens takes a severe knock, it's worth checking movements and focus to make sure nothing's out of alignment.
Preventative maintenance – basically cleaning and re-lubricating – is best done by a professional. For a time frame, I'd say every 2-5 years would be more than enough, and even 5 could be considered conservative and cautious for some users.
If you do send your lenses in for service, something to consider is whether to send the camera in as well, so the lens can be calibrated to that body. This is obviously a bit more expensive, and perhaps mildly obsessive, but if you prefer shooting fast primes wide open, it could be worth it for you.