It's individual and also depends on the game (some are much shorter like 8 hours, some can absorb 100s of hours, some games don't have a clear "end"), but one per 1-2 months with a break in between gives you high quality entertainment and it's an enriching experience.

I'd say that's a typical pace for an adult gamer with a job.

Books are a good analogy for the single player games with a definite ending. The satisfaction I get from completing them is similar (but not exactly the same).