That is the timeframe Dirk is giving himself, according to Eddie Sefko's article on the DMN website.  Some interesting excerpts:

Nowitzki also said that he's eager for something else: being a free agent. He can opt out of his contract after 2009-10 or play it out through 2010-11. He said he has no intention of signing an extension next summer when he is eligible to do so.

...

"I kind of want to see what's going to happen," said Nowitzki, 30. "I don't want to commit until I'm 35 or 36 and then [find out] I don't want to play that long. But saying that now, everything is kind of out in the open. We'll see how we play this year and hopefully bring some of the fun back.

...

"I figure I have three more years now. If we get a championship and I really don't want to play anymore, maybe I'm done. I think the worst thing that athletes can do is play too long and at the end, look slow and don't look right and they lose some of their reputation. I don't want to do that. I want to still play when it's fun."

For more, such as Dirk's thoughts on Avery and Josh, read on!