to know how long they are useful. In other words, how long
may I retain them as install DVDs and upgrade once it has
been installed and put on-line?
For ever and ever.
To expand a bit (you can tell that Brian's not paid by the word), the
numbered versions of Debian form a single chain from the earliest days.
You could literally install any numbered version and upgrade in steps
to the current one, though that would be a lot of wasted work. Any
version 9.x would upgrade in one step to current, and in the fullness
of time (probably at least a year) can be upgraded to the next major
version.
The older the 9.x version to be upgraded, the more new software has to
be downloaded, but that's still vastly less than downloading a new
installation disc, the majority of the software won't change during
the life of the 9.x release.