I was commissioned to make a wedding cake along with a dessert bar spread. They are asking for chocolate mousse to be one the dessert. My question is this does anyone know how long the mousse can sit out on display, they want me to set up the dessert bar before the reception and their guest will have access after dinner so it will sit for about 3 hours unrefrigerated but in and air conditioned room, any suggestions...

Pipe the mousse into plastic shot or wine glasses and then set into a dish filled with crushed ice. The ice will keep the mousse chilled and safe.

I agree with prterrell...I did this recently..it worked great. It reminded me of shrimp at a buffet only beautiful and delicious!!

Good idea to put the mousse on ice--but I assume you'll be doing a lot of these desserts. 3 Hours is really unrealistic to have desserts sit out--the ice will melt, the bowl will sweat...and unless you put a sign out, the guests will more than likely not wait.
If you're doing more than one dessert, I might suggest putting out the items that will hold without refrigeration (cupcakes,cookies,etc), and putting the mousse out just before service.