Not long. SUN, Birds, & Bugs love to mess with sweets. The best way to show you how long desserts will last in the sun is to have you put a stick of butter outside on a sunny day. How long does it hold its shape? That’s about your time-allowance. Your desserts will typically be set up BEFORE GUESTS ARRIVE for the party. It’s MUCH SAFER to display cake and desserts in an air conditioned space.
If you must have an outdoor display, please provide SOLID SHADE for the cake so that it does not melt in direct sun/heat.