It is hard to put a time limit on it. It really just depends on the TSO's doing the screening. Some people find it is faster, cuz it moves them to the front of the line faster.
The difference between the FB patdown and the BI patdown is just that...a FULL BODY patdown is your full body. It is the patdown you would receive if you were unable to go thru the metal detector. A BULK ITEM patdown is just the areas of your body that I mentioned (arms, torso, legs). It is less invasive and a quicker process. Then they do a complete inspection of your bag, and ETD (Explosive Trace Detection) swab the bag.