I have often said the break in period is from the driveway to the first freeway on ramp, don't baby the engine but don't thrash it either. It does take longer than the first 20 miles to break it in but it is definetly accomplished in the first 500. Make sure you use the WOT as prescribed in the manual to exert the most amount of pressure in the cylinder. The website for Moto Tune USA is geared motorcycles
Here is a differing view from another site, again pertaining to motorcycles:
"Yes there are professional tuners out there that recommend that you flog the hell out of the bike right from the start to instantly seat the rings etc & yes I will admit that those bikes have in many cases produced a little more peak horsepower initially, but what everyone fails to realize is that those professional race tuners are going to end up rebuilding that motor at least once if not twice a season (sometimes more often than that) & they don't care if engine wear is accelerated or not. Most of us want a reliable good performing motor & my break-in procedures will give you that. I've been doing this for many years & living in Memphis have seen many bikes bought in the afternoon & being dragraced that evening & they always end up smoking within about 1500 miles.
The aviation industry has without a doubt the most explicit & detailed break-in procedures for internal combustion motors. The reason for this is that they MUST be reliable as an engine failure in the air has the potential to be alot more dangerous than an engine failure on the ground. Their engine break-in regimens consist of many of the exact same key elements that mine do. Heat cycles, no initial steady rpms & ever increasing rpm peaks.
One final note is that even though the engine may be broke in properly many times the transmission still takes longer. It is not uncommon for a motorcycle transmission to need 1800-2000 miles to get fully broke-in & during that time is susceptible to accelerated wear just as the motors are. You may even notice that the more miles you put on your bike the better the transmission starts to work. During break-in keep your shifts solid & smooth, use the clutch for both upshifts/downshifts & just generally be aware that all new metal parts need time to bed in properly."
Check out this site, it relates to automotive engines with a little more detail.