Whipped cream will keep most of its volume for up to about 4 hours. Just before serving, you can give it another quick whip to "fluff" it back up.
When made with confectioner's sugar, whipped cream holds a little better, thanks to the cornstarch in the sugar. To hold whipped cream for even longer, you can add a little plain gelatin, but the result will not be quite as soft and creamy.
For more tips on making whipped cream, see our video How to Make Whipped Cream.