I made lasagna in a deep dark roasting pan and froze it. Now I am going to have to reheat it. How long in the oven?
There is no way to answer this. An exact calculation is next to impossible, anyway. An educated guess would have to take into account the temperature at which the lasagna is frozen, the setting you choose for your oven, the lasagna thickness, and the temperature at which you want to eat it, and will still be way too inexact, something like "between 45 and 75 minutes".
You'll just have to check it every few minutes, or put up with overheating it and then waiting for it to cool down to edible temperatures.
Reheating a lasagna is much like cooking it in the first place: you simply want to bring it up to serving temperature (although the second time you are not so worried about browning the top).
Because you are simply heating it through until it is hot enough to be enjoyable, assuming you have thawed it first, it is going to take about the same length of time to reheat as it did to cook the first time.
The only difference really is that you might not heat it quite hot, and you don't want to over brown the top (which can be mitigated by covering it with foil).
So I would recommend starting at about 3/4 of the original bake time, and then checking it with an instant read thermometer. Starting around 150 F it really should be hot enough to serve; making it hotter is just a matter of personal preference.
Its impossible to give an exact time, because of the many variables such as the initial temperature of the casserole, the temperature of the oven, the thickness of the dish, and so on--but you should have a good estimate from when you first baked it.