Please help me! Question about baking and broiling?

I just made a Broccoli Casserole. It has cream of mushroom soup, broccoli, cheese, a small minced onion, and some ritz crackers on top. I was waiting for my oven to preheat but it was taking too long. I check the oven and it turned out that it wasn't working. The stove and the broiler was but not the oven. My dad told me to just put it in the broiler and keep checking on it. The Broccoli Casserole usually takes about an hour, so should it be done by half an hour in the broiler? I'm so disappointed and I was looking forward to it. I'm a beginner at cooking/baking and I wanted my parents to be impressed. Basically, my question is: Since the Casserole is in the broiler instead, how long should I cook it if the usual time in the oven is 1 hour?