OK, that would be decided on the ingredients, size of dish, and temperature. For cobbler, for instance to double recipes, its add 25 minutes. For Cinnamon rolls, its only 12 minutes due to the 425 degree heat.
My best bet for your apple crisp: add about 1/2 hour, it's pretty flexible and watch for bubbling, thats a good sign it's done. Good luck!
Probably just a few extra minutes.
Probbly no more than an hour. If it's really thick (ie: you didn't use a larger dish), then it may take 75 min. But an hour is about all the crisp topping can stand.
I don't konw of any formula. It all depends upon what the item is.
Oven baking does not necessarily require increased time with increased ingredients.
If you are using a pan which will keep the depth of the apple crisp the same as a single batch, then you only need a slight addition of maybe 5 minutes to be sure the center is baked through.
Start checking your crisp at the time of your original recipe to see how much longer you need.
It depends on the thickness. If you double the recipe and also double the pan size (in square inches), the baking time would remain the same (in the example you have given).
To determine the pan size in square inches, multiply width times length. Try to match your pan size to your recipe size. If you can do this, the baking time will be approximately the same.
Other than that, I know of no magic formula, for if you change the thickness, you will surely change the baking time, and probably also the ideal temperature.
Traditionally, the accepted way of doing this is to use multiple pans. (If you have them.)