Right- this is an important question to understand better. It's hard to
define the terms well enough even for a rule of thumb- but here goes a wild
guess as a straw man. My sense is that deeply interactive works (type 2)
can usually still be resuscitated (though it isn't effortless) after about
3 generations of slumber.
Less interactive type 1 docs that are in popular formats can potentially
survive much longer- maybe dozens or even hundreds of generations. The
danger with type 1 docs is the illusion that longevity equals immortality.
Because of this illusion, type 1 docs might demand even more vigilance in
the long term.
I'm doing one test with type 1 docs now- I've declined to update my copy
of microsoft word since version 5. I ask everyone to devolve files they
send me to rev5 and I send out v5 files to others. How long will this be
possible? My guess is about 10 years. And then a lot of texts will start
to vanish- though they won't vanish all at once; v5 mword files can still
be figured out to some degree with an ascii viewer. In as soon as ten
years, if nothing is done to encourage better practice, we might find that
some archival texts have lost their italics and artful pagination.
Ascii itself would seem to be a likely candidate for a long-lived format-
although in practice it has to be housed in less long-lived delivery
formats- such as hard disks readable by varied OSs. As hard as it is to
imagine now, even ascii will inevitably start to drift, but perhaps not in
our lifetimes.
Best,
Jaron