A lot of it just depends on you as a player, and your skillset and whatnot.
For me, in games like Tetris, Smash, and Megaman, I lose skill very slowly (I run through Megaman X once every few months to a year at best and always perform comparably). But in games like Dota, more traditional fighters, or F-Zero, my skill drops pretty quickly if I don't play regularly.
I could be wrong (I'm sure there's a lot of psychology and stuff that goes into that sort of thing that's beyond my knowledge), but ultimately I think it's just a matter of your brain and what you have a knack for. Of course, if you find that your skill atrophies too quickly, a potential solution would be to just keep a regular play schedule (not unlike exercise, in some ways).