I routinely hold flasks 7 days before burnout. I hold casting
classes once per week. The first week, students make up their models
and invest them. I hold them in a cupboard for the week and when I
arrive in the AM of the next class day, I remove the bases, clean up
the sprue holes if required and put them into the oven so they can
cast upon arrival that evening.
I do not rehydrate the flasks and find they retain a reasonable
amount of moisture just from being in a closed cupboard. Our burn
outs and castings have been just fine.
Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance IS fine jewelry!