In terms of simply maintaining the current level of skill, I voted for 200 rounds a year. I would say to spread that out into 50 rounds every few months. While more is better, this number should provide to maintain skill.
Now, I certainly agree with the above posts. Particularly if you choose to minimize the number of live-fire rounds, all of your live-fire efforts need to be focused on quality practice.
There are lots of shooting skills that can be maintained (and even dramatically improved) without live rounds or even going to the range. Practicing your draw, presentation, aiming, and trigger discipline, and re-holstering can all be easily done at home with dry fire. Plus, Laserlyte (and similar) gadgets are continuously being introduced and improved upon to help with non-range/non-live fire practice at home.