Why do reusable heat packs have a limited time frame before the reaction becomes irreversible?
The sodium acetate that shows this behavior is not anhydrous $\ce{NaCH3COO}$ as you write. It is the hydrated salt $\ce{NaCH3COO·3H2O}$. The anhydrous salt does not show the phenomena of surfusion. Only the hydrated form does it. If you keep the solid form of the hydrated salt a long time, I think it will slowly loose water by evaporation through the plastic bag. So it will be partly transformed into anhydrous acetate. And by heating the whole system, it will never melt entirely. So it may never return to its supersaturated form. It is necessarily to add a few drops water to get supersaturation again.