How long to let drywall dry?

Dry is not the Word your looking for here! Cure is appropriate.. Stucco as with all Portland Cement based products depends on water in the mix reacting with the cement. As this reaction takes place the water is absorbed into the cement causing an "Exothermic" event [creating heat] and causing the sand and aggregate to bind together, basically forming artificial rock called concrete. Stucco is a generic name and denotes a fine sand mix texture but it has the same principal.

Once cured, a majority of the moisture has been consumed by hydration process and some is lost due to evaporation. Most engineers call for a cure period of 7-15 days to allow for a complete cure. Premature hydration or evaporation will deter a good cure and cause shrinkage cracking. Even after the cure period [to dry out?] stucco will absorb moisture from high humidity or rain. This makes it WET again !!

Allowing moisture transfer in and out of stucco is an invitation to a host of problems! If you paint stucco but still have not sealed it, moisture will bring chemistry out to the surface where it will blister and delaminate coatings. Commonly called "Alkali Burn". Waterproofing a stucco ASAP to control the cure, eliminate cracking and be able to paint in 48 hours, is what I suggest.