The short (and likely useless) answer is: "Whenever it needs to be cleaned."
That being said, most of my telescope (and binocular) objectives have never needed to be cleaned.
Just over a week ago I showed my son the objectives of my two largest refractors -- a 10-year-old 6-inch achromat and a 30-year-old 5-inch apochromat. The achromat's objective needed cleaned once, after a rubber retainer (split ring) came loose and deposited some residue on it. The apochromat's objective has never (in 30 years of use) been cleaned. I shined a light on those objectives to show my son the level of care (preventive maintenance) that I give them. Even with a light shining on them, those objectives were spotless and dustless with absolutely no visible dust, water spots, finger oils, etc. Those objectives are just as clean and pristine (if not more so) now as they were when I first received them as new instruments. And I'm not exaggerating!
This (cleaning) is one of the many reasons that I tend to recommend relatively small, inexpensive, achromat refractors to beginners. It's far better (my opinion) to learn how to care for a telescope by making all of one's mistakes on a relatively inexpensive, and relatively easy to care for telescope than to subject a more expensive telescope to harsh usage/cleanings.
My (not so secret) secret is preventive maintenance. I don't permit dew nor frost to form on my objectives, and that goes a long way in eliminating water spots and the need for contact cleanings. I'll generally use a few puffs from a bulb-blower on the objectives at the end of my observing sessions. Other procedures are made use of to prevent moisture condensation on objectives when taking a telescope outside, when using it, when bringing it back in, and when storing it.
There are of course details involved in my preventive procedures, but I'm old, lazy, and have other things I need/want to get to today. Those other details would be more about preventive maintenance, and not really about cleaning. Besides, those details will differ somewhat with different telescopes, used under different environmental conditions, etc. In other words, it's probably better for each person to work out the "best" procedures for their different situations -- coming back to using an inexpensive telescope to learn such things with.
But of course, the world we seem to live in (as a general rule) ignores this, and the typical recommendations dished out are to purchase the largest aperture, and/or highest quality telescope that one can afford and make use of. Afterwards, we see postings on damaged, expensive telescopes by owners who thought they knew how to take care of their telescopes. We also see many selling telescopes -- sometimes because of the damage that they had incurred.
Anyway, the short advice is to clean when cleaning is needed -- and that will of course have different meanings for different individuals; but for some it's possible to flat out avoid any contact cleanings forever, or until some unique situation arises that makes a cleaning necessary.
Note: Just because I avoid contact cleanings (except in unavoidable situations, such as with that previously mentioned O-ring problem) does not mean that I permit my telescopes to become dusty, etc. Again, I employ preventive measures to keep my telescope optics in pristine condition -- without the need for contact cleanings. It's possible (at least for some of us) to maintain pristine optics without ever cleaning them. And if one can do that, then I think that ought to be the preferred approach. It's nice, when taking out and using a telescope, knowing that the optics in that telescope are as clean as they can possibly be -- each and every time the telescope is taken out to be used.
I'm not worried yet but I might be soon, maybe after this summer...
