Leftover lasagnas can be refrigerated for 3 days or frozen for up to 2 months. When ready to reheat, remove the lasagnas from the freezer and let sit at room temperature for 1 hour to thaw, then bake in a preheated oven at 350 degrees F for 15 minutes. You can also bake them directly from the freezer; just increase the baing time to about 30 minutes.