Oven – Why does the oven take so long to heat up

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.

Since it's already fully-cooked, you don't need to be all that precise with time or temperature. The most important thing is to avoid drying it out, so use a roasting pan and add a little water to the bottom (not more than 1 cm) and cover it while it's reheating.

It's best if you have a meat thermometer; toss it in at 300° F / 150° C (or up to 350° F / 175° C if you're in a hurry) and aim for a temperature of about 140° F / 60° C. If you don't have a thermometer, this about.com page suggests 16-20 minutes per pound; it also suggests heating to 135° F which is also fine. But if you're not going by a thermometer then you'd better make sure that it's completely thawed first, all the way to the interior, lest you end up with a semi-frozen dinner.

If you also plan to glaze it then apply the glaze after it's been heated and raise the temperature to 400° F (205° C) as the above link suggests.