Fergy's approach has it just right for me.
As short a time as it takes to bed in the brushes is ALL that is required.
Any more than that simply reduces the motor's life to NO gain whatever.
Submersed in liquid is fine, and again he has it right - it takes less than a tenth of the time that it does in air. But is it worth the bother and mess and the real risk of OVERdoing it? I really don't think so. Any running-in time saved is usually expended again on removing and replacing the motor, setting up the witch's cauldron and cleaning up afterwards. If you overdo it, your motor is probably screwed because most modern motors do not have replaceable brushes.
On balance, wet bedding is definitely not worth the bother.
How do you know when the brushes ARE bedded in?
Ideally by checking rpm with a rev counter, either a stand alone or on a rolling road (Kelvin Test Bench). It doesn't take long to ascertain when the motor is not improving any! Lacking a meter, then simply listen to it - the old fashioned way. The higher pitched the whine, the faster it is running, but a rev counter is better.
Just an observation on rpm, reliably ascertained using a Kelvin Test Bench.
Newly acquired cars are sometimes not well lubricated - motor bushings, gears or axle bushings. In these cases, on average , the simple act of lubrication can increase rpm (and decrease required current!) by around 10%.
In some cases by more than 15%!
Many cars ARE perfectly well pre-lubed and, again, a rev counter will soon tell you whether there is no improvement from lubing. Another tip - wick up excess oil with torn paper or a cloth.
Enjoy! 
