At 350 degrees you usually cook the turkey for 20 minutes per pound, but at the low temperature of 250 you'll need to cook it for 6 to 8 hours. Check with a meat thermometer a â¦ nd make sure if reaches 165 or higher in the deepest part of the turkey. 28 people found this useful. basic lasagna typically has approximately 4 layers of noodle â¦ s, cheeses, and sauce. So, based on that figure, it would take approximately 1 hour at 350 F in a convection oven. In a regular, conventional oven, it would take approximately 1 hour and 15-20 minutes in a preheated 350 oven.