just ask a number of instructors in the flying club when you should lean a Lycoming engine (answers below please)
I've had:
"Actually we don't mind if you never lean, which is why we don't teach it to students. It's lots cheaper for us if you just waste a bit of fuel than if you over-lean and we have to repair the engine."
With the result, of course, that when one does ask to be taught leaning later, putting the mixture back rich again is not an automatic part of planning or performing a descent ... but at least they taught me "mixture fully rich" in the downwind checks from lesson one, so I've never actually landed still leaned.