If your definition of “hard” is “too hard to scratch into”, the
answer is never.
It acts much like plaster, but it really isn’t plaster at all, and
it doesn’t act exactly like plaster in all respects. (which is why
we use it for situations that plaster would never tolerate.) One of
the differences being ultimate strength. Even when fully set, it’s
not all that hard. You can carve it with a butter knife. (Which is
what I use to trim my flasks.)
I’m not sure exactly what you’re asking, but I suspect you’re trying
to figure out at what point it’ll be too hard to scratch casting
info into? Never.
It’s got a shock sensitive stage from about 9 minutes after the
water hits it until about half an hour later, so do not handle it
during that phase, but after that, you can carve whatever info you
want into the base of the flask, using pretty much anything. Pencil
works well, or an old nail.
Hope that helped.