How much practice does it take to keep the shooting ability on par? How much does it differ with different types of firearms?
Not necessarily topnotch ability, but sufficient for regular shooting.
IMO, once you become proficient, with handgun or rifle, it becomes muscle memory.
Maintaining physical condition to execute that muscle memory becomes the issue after that.
I find I can stay very proficient with a rifle pretty much indefinitely. Handgun skills drop quickly.
Winshooting and action pistol disciplines require practice for me. If I miss a few weeks of pistol dry fire drills my performance drops off hard. The first few pigeon shoots of each season usually show up my lack of mounting and swallow and bat tracking sessions in the garden.
Generally speaking, my shooting seems better the more I shoot. Groupings, speed, comfort etc are all much better when I get to the range the second time in a month.
An old soldier (who could really shoot) told me it is way more effective to shoot ten shots every week than 40 shots once a month. He could hit a golf ball with almost any handgun at 15m.
I'm inclined to look up at my target when firing. To get rid of that, I go out for a couple of practices before the hunting season. As much as I know my reloads, I go "check" them. I do that thing Ds J refers to of rather shoot a few shots on more occasions than a huge bunch once.