No bake cheesecake needs to be refrigerated for at least 6 hours before it starts to firm up. The longer it's in the refrigerator the better, preferably overnight. I find day 2 the cheesecake is at its firmest. Also, when making your cake make sure your whipping cream is whipped to stiff peaks and is folded into the batter. You want to make sure you keep the air in the whipping cream, avoid stirring it into the batter. Also, if the strawberries are too juicy they can throw off the ratio in the recipe, causing too much liquid. This makes it more difficult for the batter to set.