How long chilled lasagna takes to cook depends mostly on the size of the pan, as well as the oven temperature. Preheat the oven to 400 degrees Fahrenheit. Bake an 8-inch square pan of lasagna for 40 to 50 minutes. A refrigerated lasagna in a 9- by 13-inch baking pan will need between 55 and 70 minutes, while a jumbo-sized pan needs 70 to 85 minutes. Regardless of the type of pan you use, cover the lasagna loosely with foil during the first 30 to 50 minutes of baking. This ensures the lasagna heats evenly without drying out. Remove the foil during the last 20 to 25 minutes of baking so the cheese becomes bubbly and golden brown.

Another factor that can alter cooking times slightly is the ingredients you use in the lasagna. A lasagna with many layers takes longer than one with only three or four. Heavy, dense ingredients, such as thick layers of ricotta cheese and meat sauces, add to the cooking time, as well. Shredded cheese cooks more quickly than sliced cheese. Add an additional five minutes or so to your cooking time to accommodate a thick, dense lasagna.

A golden crust, tender pasta and bubbling sauce are all good indicators a lasagna is cooked through, but the only sure way to know is to measure the temperature. Bake a refrigerated lasagna until a meat thermometer inserted in the center of the pan registers 165 F. Cooking a lasagna destroys pathogens or bacteria lurking in the casserole.

How long a refrigerated lasagna takes to bake also depends on whether it's partially or fully frozen when you put it in the oven. A frozen lasagna can be cooked directly from the freezer, but it takes twice as long to bake, and it may heat unevenly. A better solution is to thaw the lasagna overnight in the refrigerator. A lasagna that is still partially frozen when you slide it into the oven will take longer than a fully thawed one. Never thaw frozen lasagna on the countertop, which leads to bacterial growth that can cause foodborne illness.