The fruit layer will begin to bubble or fizz when it is in the oven. It is probably good to go when you realize that most of the apple crumble is a medium brown. Although all ovens work differently, about 45 minutes in a preheated oven will have the crumble ready. Keep your eye on the stove, though, whenever you are baking. Since it’s your first, the time taken for your oven to make this crumble can be used as a reference in the future.