ROI depends on level and speed/structure.
I'd say you want to play at least 400 regular speeds or 600 turbo speeds in any given month.

Assuming you are +EV in all of your games, more games will generally result in more money. If you are just looking to make money then you should probably be playing as many games as you can handle. Just be sure that you aren't grinding it out while playing poorly because you could be -EV if you're really straining yourself.