Debian v.9.2.1 DVDs - how long are they good for?

[Nomen Nescio <nobody@dizum.com>]

I downloaded some Debian v.9.2.1 DVDs and I would like
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?

Also, how often would I need to replace/download new versions
of the .ISOs (to ensure I could update/upgrade once installed
and on-line). I know there are other ways to go about this,
but I prefer to carry these install DVDs to make it easier
for installing onto my systems and clients. TIA.