if your freezer is working great and you wrap the lasagne properly i would say a couple months. i have kept lasagne in my freezer for 6 months. when i took it out and heat it up, it was just like a fresh one, no difference.
roshay, if you are asking about the refrigerator, I would say no more than 3-4 days. Also, when you heat it up, only heat the portion you are going to eat, do not heat the whole tray (unless of course you are feeding the whole family again) and then have it cool down and heat up again another time.