When will Ubuntu 18.04 release and at what time?
Each time it is different :-)... and unknown in advance (at least until the 18.04).
Update: (Thu Apr 26 22:59:59 UTC 2018)
Now it's out. You can start from the Bionic Beaver page or from the links below.
One of the most comfortable solution to know when a new release exits, is to subscribe to the ubuntu-announce mailing list and wait for a mail. (As you can check there are not so many and it will not spam your account).
It will appear in the Bionic Beaver (18.04) page instead of the beta-2 version, or in the download sections of the Ubuntu site [d1, d2], or on the cdimage Ubuntu site in the release directory (again not the beta-2)...
Nonetheless you may have some hints from the past [12.04, 14.04, 16.04],
looking for the 1st and last ISO file creation time present in the old-releases Ubuntu site:
Release Day 1st ISO file Last ISO file Last FILE
12.04 LTS 26th '12 Apr 23 12:27 '12 Apr 25 16:13 2012-04-26 09:51
14.04 LTS 17th '14 Apr 16 21:13 '14 Apr 17 01:37 2014-04-17 15:00
16.04 LTS 21st '16 Apr 20 22:30 '16 Apr 20 23:38 2016-04-21 11:08
Of course you have to search for the first release (the one without .1, .2, ...).
For a somehow more "trusted" version (suggested as automatic update from the previous LTS) you should usually wait until the .1 release (usually in July-August; this time it is scheduled for July 26.
We should mind the timezone and that we should even wait until the files are uploaded to the various mirrors.
According to this, this is a bug that delayed the release of Ubuntu momentarily.
About seven hours ago, the development team found a bug and it’s currently being fixed. Apparently, Bug #1767067 affects the live session options in Ubuntu, Ubuntu Kylin, Ubuntu Budgie, and Ubuntu MATE flavors. As per the latest update, the team is working hard to fix the bug and testing the release. (Update: The team has fixed the bug and the official release will be out anytime soon).