How long to preheat oven?
This is a silly question, but my wife will turn on the oven to preheat while she prepares stuff, but sometimes it takes her 15 minutes to 45 minutes before she puts things into the oven.
Generally, how long does one need to preheat the oven before it's to the desired temp? I know that this can easily be solved with a thermometer, but we don't have one.
This depends on a lot of things.
The idea of preheating is that you want to get all the surfaces inside your oven (walls, floor, door, racks) up to the desired cooking temperature. This makes for more even temperatures throughout the oven, and gives a little thermal mass so you don't lose ALL your heat when you open the door for a few seconds or put something cold in there.
Then there's the question of what you're putting in the oven. An aluminum sheet with a few room temperature cookies on it won't pull the temperature in the oven down like a 25 pound turkey that's 40F/5C inside. You want to be more careful to do a complete preheat if you're going to be soaking up a lot of your starting heat.
Our oven, which has a large baking stone in the bottom all the time, takes a while to get uniformly up to temperature, even after the oven says it's preheated, because the stone doesn't heat up as fast as the rest of the surfaces. It takes at least 20 minutes after the "I'm fully heated" beep before the stone is fully up to temp. We have problems with things baking poorly if we don't preheat for quite a while, but on the upside, if we put a cold roast in or open the door a lot, the temperature in the oven stays pretty high.
If your oven is lightweight, flimsy or drafty, it may be as hot as it's going to get the moment the preheat alert goes off.
45 minutes is probably a lot more preheat than you'll need in almost any case. In some cases even 15 minutes is more than you need. It really depends on your oven and what you're putting in.
Absolutely correct. The preheat beeper goes off when the air has reached temp, and opening the door of the oven will kill that right away. The more thermal mass (baking stone, bricks, I've even seen a full clay oven liner(found it http://www.hearthkitchen.com)) you have the longer it takes to heat up, BUT, the less fluctuation in temperature as you put food into it.
Add to that... consider the degree you *need* to maintain constant temperature. When baking bread, you don't want that initial drop in temp from improper preheating (so preheat longer). But if you're warming up yesterdays casserole, who cares?