Rewritten for accuracy/clarity.
The reason they say to bed in the pads initially (right after purchase) very slowly over the next couple hundred miles is not really bedding them in. They say this (and rightfully so) to ensure that the pads mate properly to the rotors -that is, the entire surface of the pad is in contact with the rotor surface- before you use your brakes hard. If you do any hard braking when your pads are not mated to your rotors only part of the pad surface is touching but you are demanding the same amount of friction from a greatly reduced surface area .... you are going to get this portion of the pads and rotors extremely hot while the untouched surfaces do not heat. This can be all sorts of bad. This is especially important if keeping the same rotors and just changing out your pads. Once this mating process is complete you can then "bed in" your pads/rotors. You should be able to determine this by looking at your rotors directly after some medium braking, if the rotor still has concentric darker stripes of pad material mixed with areas of no material you still need to mate the pads to the rotors.
For bed in, it does depend on your brake pad material - its operating temp range and Mu. Begin by warming your brakes with some light to medium braking so as to not heat shock any of the brake system. Then, generally about 8 80-20mph consecutive braking (at about 8/10ths of your braking threshold) should be good for a track/street pad (two fewer for a street pad); and you, with a little work, should be able to find a place to do this, perhaps late at night on an empty highway. After you have done this, quickly find a place to drive around slowly for a bit to allow the brake system to cool somewhat gradually, then stop the car, leaving the brakes off -including parking brake- and let it sit for 20 minutes or so. Do
not immediately stop the car after the procedure. If your absolutely have to stop; come to your stop and immediately/continuously roll the car back and forth at least one wheel rotation until you may move again. Otherwise you run the risk of welding your pads to your rotors or at the very least causing uneven pad deposits on your rotors. Bedding your pads chemically bonds the pad material more effectively, giving it better wear and consistency properties.
Pads have two basic methods of operation: abrasion and "friction". Street pads almost always operate in the abrasion mode and abrade through the pad and rotor material, just the two surfaces rubbing and wearing against each other. Track pads are designed to operate most effectively in the "friction" mode. This is where a
consistent layer of pad material has been deposited on the rotor surface (due to heat) and the pad material on the rotor and the pad itself are what is rubbing against each other. This mode provides both better stopping power and linearity than the abrasion mode and doesn't directly wear the rotor surface. Thing is, if you run a track or track/street pad on the street and constantly operate your pads in the abrasion mode you will be causing greatly increased wear on your rotors than you would with a street pad due to the higher coefficient of friction (Mu) of the pads.
