There are two answers:
The first answer is- "Right away, if you're looking for the best results".
The second answer is- "Accu-Cast alginates lose water MUCH more slowly that traditional alginates. We have added a water retention agent which makes a somewhat delayed pour-up possible. Its still better to pour your molds sooner rather than later, but its not as much as a panic as in the old days.
An alginate mold begins shrinking almost immediately. At first the shrinkage is very small, but it is cumulative and non-reversable. We're always looking for our customers to get the best possible results, which is why we say "GET IT POURED !!!"
We recommend that you consider the "mold-making part" and the "pour-up part" to be one procedure instead of two if you possibly can.
If you're still reading this, you might want to know WHY alginate shrinks and what influences shrinkage. Here you go.
Alginate is, on a microscopic level, just like a wet sponge. There is really nothing holding the water IN the system. Water evaporates off the surface and is immediately replaced by the water just under the surface, etc., etc., etc. This is knowns as "transpiration". Another way water leaves the system is through syneresis. Imagine the mass of alginate being a water balloon, with millions of very, very small holes in it. Just the pressure of the weight of the alginate mass "pushes" water out through these holes.
The upshot of all this is that the water leaks out of the "sponge" and the alginate mass inevitably shrinks. It will eventually shrink to about half its original size and be as hard as a rock. Shrinkage also does not happen uniformly. thinner sections shrink faster than thicker ones, so distortion is inevitable. Keeping the alginate in a 100% humidity environment. Covering the surface of the alginate with a wet towel dramatically slows the evaporation, but not the syneresis.
Actually, evaporation is much more pronounced in "Layup Molds" than in "Bucket Molds" because they have a much higher "surface area to volume" ratio. These are exposing a much higher percentage of their volume to the air, so the evaporative effect is greater than it would be for a "Bucket Mold".