Chilling the Lemon Blueberry Tiramisu overnight is recommended to allow the flavors to meld together and for the filling to set correctly. However, if you are short on time, you can serve the tiramisu after it has been chilled for at least 4-6 hours in the refrigerator. You can also add a Tablespoon of pudding mix to make the filling a bit firmer, which can shorten the chilling time. Remember that the longer the tiramisu chills, the better the flavors will develop, and the more cohesive the layers will become. If you serve it too soon, the filling may be loose, or the ladyfingers may be too dry. So, while it is possible to serve the tiramisu sooner, chilling it overnight will yield the best results.