Hi Hans,
I clean them as needed, and that depends on how often you use them. 12 to 15 years ago that was about once every few months. With COVID and getting older and a lot less observing these days probably every 12 to 18 months.
How I clean them depends on the eyepiece. a 4mm ortho with a ~2mm diameter eye lens clearly has different cleaning constraints compared to a Pentax XW with a ~ 25mm diameter eye lens.
I usually blow the loose debris off with a blower bulb (Don't use compressed cans). With eyepieces having a larger eye lens I use Isopropyl alcohol on soft lint free cotton cloth patches cut into ~15mm squares. I then drag that around the eye lens with a Cotton Bud. With eyepieces having smaller eye lenses I use Isopropyl Alcohol on a cotton bud or a much smaller cotton patch and work that around with the blunt end of a round toothpick snapped in half, making sure the cotton is always in contact with the glass.
You can use whatever grade of Isopropyl Alcohol you like but I find the 100% stuff dries faster and cleaner. It's not expensive if you buy it from the right place.
5 litre bottle of 100% Isopropyl Alcohol from Sydney Solvents for $24.00. I bought a 5 litre bottle from them about 8 years ago for the same price and I still have about 1 litre left. I am about to buy some more.