Unfortunately, this is a little bit of a "How long is a piece of string question because it depends on the conditions in which the chain is used. For instance, if you are riding in the rain a lot, the chain will need Wet road conditions, or very hot and dry weather both
However, if you force me to answer, I'd say every:
Some lubricant makers recommend sometimes recommend to leave the bike for a couple of hours, or even overnight, before riding again. This gives the lubricant time to spread into the tiny gaps in the chain bearings.
This is a good idea if you can discipline yourself to apply the chain lube after the ride.
However, for the most part ...... "Ain't nobody go time for that!"