That's why they're professionals. I know someone has the exact details of how this sort of thing is done, but I take it they record as much as they can in a session per show.
Tom's one of my favorite current voice actors (though, frankly I have a bunch of them), and the only time I really feel he was overused and stretched too thin was "My Gym Partner is a Monkey," where casting him as said monkey was too obvious. Then again, the show was just overall bleh. I'm surprised at his range, though. While he mostly does crazy wacky characters like Spongebob and Ice King (who is dark, but still manages to always be goofy, which somehow makes it darker), I love his interpretation of Doc Ock in Ultimate Spider-Man. Say what you will about that show, but that's got to be the darkest, creepiest versions of the Doc to date.