The initial chilling time, at least 6 hours, is important because the graham crackers need time to soften and absorb the frosting. If you serve it sooner, you will still have parts of the cake that are crunchy. Usually, I make the cake the night before I am planning to serve it. The cake can be made up to 2 days in advance, but it will still be good for about 4 days. It is best served 1-2 days after preparing it.