When does a new version of Drupal get released?

Each version of Drupal goes through three phases of development:

1. Code thaw: Development is open for business, and anything goes. Re-write some entire underlying subsystem. Put in new, crazy APIs. Add that killer feature you've been missing from core. Code thaw normally lasts around 6 months, but with the the addition of test coverage to Drupal core in 7.x, which reduce the chances of critical bugs creeping in unnoticed, the plans are for this to be extended in the future.
2. Code freeze: Development gets "locked down" and shifts to bug-fix only mode and minor usability enhancements. No new features are allowed, and at this point, core is deemed safe for contributed authors to begin porting their modules and themes. API changes will only happen if critical bugs arise. This period will last at least 3 months to give contributed authors time to complete porting.
3. Beta/Release Candidate: The next Drupal version is gearing up for release. The goal is to get the number of remaining critical bugs down to 0. Once a RC is released with no new critical bugs found, the next version will be Drupal N.0 and release will happen.

A frequently asked question is, "Why was Drupal X released when some of the critical contributed modules weren't ported yet?" There are several reasons for this:

• Find bugs not encountered during testing: There are sometimes bugs, and even fundamental design flaws, that don't come to light until people start actually building sites on the next version of Drupal. This doesn't generally begin until the next Drupal version is finally released.
• Keep core developers engaged: Since developers can only put their wild and crazy ideas during "code thaw," which only lasts a short period of time, it's important to make sure that code thaws come around often enough for core developers to remain interested and thereby keep Drupal constantly improving.

If you think it's too premature to release a version of Drupal, there are several things you can do:

• Port modules yourself: The Coder module makes the process a quick and easy copy/paste/modify exercise for many modules. You can view a video that shows you how. The Deadwood can help a lot in this process
• Testing: Test Drupal thoroughly while it's in beta/RC status. If you find remaining critical bugs that haven't yet been found, that will further delay its release until they are fixed.