How long does cooked lasagna last in the refrigerator?

This completely depends on whether it's frozen, just made, cooked, uncooked, ingredients, etc. Your question isn't specific enough to answer properly. HOWEVER, let's assume its' a cooked meat lasagna and you had it for dinner but you forgot to refrigerate it before going to bed. If you live in a cold climate and the house is chilly all night, you'll probably be okay if you put it into a fridge immediately when you wake up. If you live in Arizona or Guam, you've just lost your lasagna. The meat and the pasta will start to grow things you can't see within a couple of hours of sitting out of the fridge. Personally, I wouldn't chance eating a lasagna that sat out all night unless I was absolutely broke and cramming for a test or so hungover that I couldn't get to the store. Then, it usually doesn't matter as long as it fills the void until something better comes along.