Rival Jay Leno is scheduled to surrender the "Tonight" reins next year.
"The way I feel now, I would like to go beyond 2010, not much beyond, but you know, enough to go beyond. You always like to be able to excuse yourself on your own terms," Letterman said in an interview in Rolling Stone magazine.
"If the network is happy with that, great. If they wanna make a change in 2010, you know, I'm fine with that, too," Letterman said.
Letterman, along with Chris Rock and Tina Fey, is featured on the comedy-focused cover of the Rolling Stone issue out Friday.
Letterman, 61, questioned why NBC is proceeding with its plan to remove Leno, who consistently tops the late-night ratings. Conan O'Brien will take over "Tonight" in June 2009, with Jimmy Fallon moving into O'Brien's "Late Night" chair.
"Unless I'm misunderstanding something, I don't know why, after the job Jay has done for them, why they would relinquish that," Letterman said, adding, "I have to believe he was not happy about it."
Letterman speculated whether "that's actually what's going to happen," while acknowledging NBC might be too far down the road to retreat.
NBC is angling to keep Leno, 58, with NBC Universal but the late-night king has indicated he's ready to jump ship. Eager NBC competitors, including other networks and syndicators, are prepared to help him make the leap.
Letterman, who called O'Brien "a very funny guy," was asked about facing him as the new "Tonight" host. A cautious Letterman said he couldn't predict the outcome.
"It will be weird to see Conan at 11:30, don't you think? Which is not to say he can't succeed, but, no, I don't know what the competition will be like. I hope we're able to do OK."
In the Rolling Stone article, Letterman discusses guests including Madonna, Oprah Winfrey and Howard Stern, with the most moving remarks about musician Warren Zevon, who appeared on "Late Show" shortly before his 2003 death from cancer.
Letterman recalled his "heartbreaking" meeting with Zevon in a dressing room after the show.
"Here's a guy who had months to live and we're making small talk. And as we're talking, he's taking his guitar strap and hooking it, wrapping it around, then he puts the guitar into the case and he flips the snaps on the case and says, `Here, I want you to have this, take good care of it.' And I just started sobbing.
"He was giving me the guitar that he always used on the show. I felt like, `I can't be in this movie, I didn't get my lines.' That was very tough," Letterman said.