how long will this sit?

Hi Mark! This mousse actually gets thicker as it sits, so my only thought would be: When you whisk the chocolate mousse, make it it a little looser than you'd like it, since it will continue to thicken in the fridge. If you want to serve it for dessert, making the recipe that afternoon would work. You can portion the mousse in individual dishes, or layer the two flavors in a big bowl and then scoop into individual dishes right before serving. Hope this helps!