Definitely give yourself enough time to let each layer set. I gave the blue jello about 45 minutes to set, then i let the whipped cream layer set in the fridge for another half hour or so. I found the red jello didn't really need to be set. The whipped cream was solid enough just pouring the gelatin liquid on top---slowly, and by spoonfuls--it worked just fine. I let it sit in the fridge over night, decorated with berries and whipped cream before our bbq and it served great!
So, definitely give yourself enough time to let each layer set.