On my commuter bike:
Once every 2 months, using cleaning Benzin or cleaning Gas (funny maybe, its cheap and works wonders - not sure whether I am translating this right though...) with ParkTools chain cleaning tool.
Re-lubing with any cheap lube I have. The bike does 35-40km per week, continental climate.
On my road (fun) bike:
Whichever comes first - every 500km or 1 month. Usually during bikepacking trips I wipe off dirt as best as I can (think paper/rag found on the side of the road etc.) and add lube every 4 days or
as soon as noise gets too high. For fun bikes that stay on the wall and rarely have fun I apply Liqui Moly dry lube and let it...dry...for at least 5 days (I do wipe off excess thoroughly).

I do get lazy very often so I usually just oil my chain when I feel like it. Good therapy.

Do not follow my example, its generally stupid, find your own flavour.