The normal temperature for cooking lasagna is 350 degrees for about 30 minutes. Going from 350 degrees to 250 degrees is a 71% reduction. 71% of 30 is 21.3.I would conclude that it would take 51 minutes. 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.