This all depends on what iOS version it is running.
If it comes out with version 5.1 those who are into making jailbreaks have already had time to start working on the jailbreak and if they don't already have a working jailbreak it shouldn't be long after that.
If it is version 6.0 like some websites are finding in their server logs, it could be anywhere from right after the launch to a month or two. There is no real answer however I don't think any device has gone longer than a month or two before having a jailbreak available.
the key is that its less dependent on the device and more dependent on flaws in the version of iOS that it runs. While iOS 5.1 is not publicly available yet, its freely downloadable for app developers and can already be looked at.
