Time in the dehydrator depends on the size of the space you're trying to dry as well as ambient humidity and drying temp. I most often dry on 95F and can keep cookies in there anywhere from 10 minutes to 30 or more minutes with little to no effect on the cookie, though about 10 to 15 minutes is my norm.
Some exceptions: I tend to keep them in longer (closer to 15-25 or so minutes at 95F) if the areas are the types that are prone to cratering (small and angular). I also sometimes ratchet up the temp to 105F if I have very large areas/topcoats to dry (like over 5 or 6 inches). I find that they can sometimes dry with a ripple at the lower temperature, but they very consistently set faster and smoother at the higher temp. To note: with 15 to 30 minutes in the dehydrator, my icing is rarely to never dried all the way through; it's just crusted and set to the extent needed to prevent cratering. So you'll likely need to dry longer outside of the dehydrator if you're planning to heat seal/tightly package.