I am consulting on a soon-to-be-launch Drupal project. When development started a few months ago, the first key decision was on which Drupal core version will be develop on. It was decided that Drupal 7 is not yet mature enough for our needs. Many key contrib modules were not ready, including views and panels. Our key developer also felt that performance is not that great, although usability is much improved over Drupal 6. In the end, we were able to develop most of the functionalities based on what Drupal 6 has to offer.
Now, with the site nearly its first release, we start to look ahead and I again think about the Drupal core version. Will we be forced to migrate the site to Drupal 7, and when? It wouldn't be the end of the world, but probably not a good idea either to run an unsupported version of the CMS. So when Drupal 8 comes out and the support for Drupal 6 is discontinued, we will not have much choice but to make the switch.
I've been using Drupal since 4.3, and the development cycle has always intriqued me. Sometimes, it happened very quickly. (Drupal 5.0) Sometimes, it takes more than a year from code freeze to point zero release. (Drupal 7.0) Since today is still a holiday, I indulged my curiosity and created this chart after digging through the releases page and forum announcements.
With the vision for Drupal 8 still taking shape, and if the core development timeline is somewhere between those of Drupal 6.0 and 7.0 release, we should have at least a year and a half of security support for Drupal 6.
Don't get me wrong, I'm sure Drupal 7 is a fairly good product, and Druopal 8 is going to be awesome. (I'm especially excited with the Mobile Initiative.) But migrating large and complex site across major Drupal core version is not something I wish for my enemy.
P.S. For those not familiar with Drupal core's release cycle, there is an explanation page.