For both Lion and Mountain Lion it was right around 4 weeks from DP1 to DP2, then about every two weeks until Gold Master. Keep in mind that in neither of those cases was DP1 released during WWDC, so the engineers being busy may delay it further than 4 weeks this time.
Logically, it make sense for the DP2 to take much longer than the rest of the DP releases. Apple's now getting reports from all of the developers and will be going through all of them to fix as much as they can for DP2 update. It is going to take some time to organize where the fixes are prioritized and so on.
After that, the amount of fixes they have to do will decline (4 weeks of bug fixes) and it'll stay at 2 weeks per update after that until they're ready to release it.