We spent about 4 hours at the science center on a recent trip to Toronto. Four hours was definitely not enough-the place is huge and full of amazing interactive exhibits. We didn't even get close to seeing the whole thing.
The day we went there were a ton of field trip kids there and it was pretty loud and hectic with long waits to get to some of the more exciting exhibits but they were worth the wait.
If I every get back to the Toronto area another trip to the Science Center will be on my must do list!