Step-By-Step Cookies: Crayon Banner Cookie 

Hi! You can certainly air dry or use a fan to help the icing dry faster which will also improve the finish of the icing. When you look at your icing from the side, it will no longer appear wet (super shiny) and that's the easiest indicator that it has crusted. It only takes a short time in a dehydrator, a little longer with a fan and much longer if just air drying. It also depends on your icing and humidity level in your home, as far as time it will take. I hope this helps!