Well, the plan is for the game I'm currently playing in to stop next summer, so the GM can take a break if not end it on some big climax. He's been running for a couple years now, he deserves a break. I've agreed to run 4E and put it through its paces, so we can decide whether to continue 3.x or move on to the next edition. Whether we'd keep the same characters and re-imagine them, or move on to a wholly different campaign, is still up in the air. So, my answer probably SHOULD be 'immediately' - granted nobody makes a prima facie case the new rules suck within the first few days of its release.
However, it looks like I may be studying abroad in Europe around the time of release, sooo....maybe not. It might be more like a month or two.