There is no exact time when Ubuntu is released.

You can find the expected release date on the Release Schedule. In an interview on Ubuntu-User Kate Stewart, the Ubuntu Release Manager at Canonical, outlines how the release announcement works:

The release team needs to do final checks to make sure all the right bits are in the right places, and no blockers have cropped up as a result of the last bug fix or upload. If people are trying to download the images during this final bit of testing, the slower the testing goes (and they run the risk of pulling down a flawed image) and it delays the time until we can actually release.

The official release announce is mailed out to ubuntu-announce maillist. Copies of prior release announces can be found in the mail list archives. Once we hit send on that email, the IRC channel is updated, as is the News & Announcements section on the Ubuntu Forums, a post is made on the release blog, and the Launchpad project is updated.

The simple explanation is, it's not based on time, it's based on the amount of checks they get through.

Recently the team has added a realtime checklist that you can follow along with.