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.
cooking.stackexchange.com/users/3348/bikeboy389
Most ovens I have used take from 15 to 20 minutes to get to 180C (350F).
Many ovens have an indicator light that glows while it is below temperature (i.e. when the elements are on) so just watch for that to go out, and your are at your desired temperature
The key thing is that most modern ovens are well insulated and will not use much energy once at temperature and with nothing else to heat up, so going a bit longer isn't a big energy waster
If you want to save energy have a queue of things to bake one after the other, as a significant amount of energy if used just getting the oven up to temperature.
Electric ovens are from 3000 to 5000 Watts, so each warm up cycle uses approximately one kWh (assuming 15-20 min)
cooking.stackexchange.com/users/3203/TFD
My electric oven takes 25-30 min to reach 350 F. You might not want to trust your on-board thermostat, though.
I recently got a new oven thermometer and checked the temperature after the oven beeped saying it had reached 350 degrees but the thermometer only read 250.
cooking.stackexchange.com/users/37242/Dee
My sister-in-law's oven takes 20 minutes to heat up to 350 degrees.
I timed our oven last night and it took 7-1/2 minutes.
I was surprised it heated up so quickly.
cooking.stackexchange.com/users/26639/user26639
What this really depends on is what the voltage is to your house and what the oven is designed for.
If you are finding your oven is taking a long time to heat up
you will probably find that you are getting a 208 volt service (common in North America), especially in multi family buildings.
If the oven is designed for 220-240 - you will notice a significant slower heat up time.
North American made products are typically designed to work with both volt supply types.
My manual for my oven (Fisher&Paykel DCS - from New Zealand) reports the oven will heat to 225 C in 15 mins - it actually takes 39 minutes....
super annoying.
cooking.stackexchange.com/users/None/
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.
How long to preheat oven?
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cooking.stackexchange.com/users/2125/milesmeow
It can take up to half an hour or more to reach the right temperature, depending on your oven and whether you are using a baking stone. Most modern ovens have good insulation, so not much heat and energy will be wasted once it has reached the desired temperature. It can take quite a while to preheat an oven to the right temperature. Ultimately, it depends on your oven and what you are cooking.
It can take longer than you think to pre-heat an oven. 45 minutes is probably too though. It depends on your oven and what you are going to be cooking. Leaving it to heat for longer than you need probably won't waste much energy.