How can we predict when the next Debian release will be out?

See Debian Release Management; for Debian 9, it stated:

As always, Debian 9 “Stretch” will be released “when it’s ready”.

and that’s the general rule for all releases.

The planned release date for Debian 9, June 17 2017, was announced on May 26 of that year. The planned release date for Debian 10, July 6 2019, was announced on June 11 of that year. (Both releases happened on the planned date.) Debian 11 is currently being frozen, and although the full freeze date hasn’t been announced yet, another summer release seems likely.

Generally speaking, you’re right that “when it’s ready” correlates to the number of (release-critical) bugs in the testing distribution, to a large extent. The release team give regular updates on debian-devel-announce, which are linked from the release management page. These updates list the items which still need to be fixed (including bugs, but not only), and explain how you can help; that’s mainly:

• test the current testing distribution;
• help triage bugs;
• help fix bugs.

The best way of knowing when a Debian release will happen is to help fix the issues preventing it — as the number of such issues goes down, so does the release date get closer.

You can track the release-critical bugs; those which matter for the next release are counted as “number concerning the next release”. Other important ingredients for a Debian release are its installer and its documentation.

I know you've already chosen your answer but this may help. From past experience full release is roughly 7-8 months after full freeze, which would mean that about 40% of the release critical bugs have been fixed at this point. Current expectations in the community are that Stretch will go stable at some point before September. However, a release date announcement is expected soon.

For more help on this topic, read this post:

http://forums.debian.net/viewtopic.php?f=20&t=132885#p642644

and this

https://lists.debian.org/debian-devel-announce/2017/04/msg00013.html

You can give a try to my silly guess script:

$ ./debdatez 
['19   July        2002',
 '6    June        2005',
 '8    April       2007',
 '15   February    2009',
 '6    February    2011',
 '4    May         2013',
 '6    July        2015',
 '22   June        2017']
average time to release Debian: 67293257.1429s
...or: 2 years, 2 months, 18 days
so next release is definitely on 09 August 2019
$ 

In case it's not obvious, there's no ambition to be preise: It merely computes average time between releases and makes "prediction" based on that. Good enough to for eyeballing when to start to be curious, though.