The charger should be designed for the idle current of the droid plus the charge rate of the battery. Many times a lithium ion battery will be rated at 0.5C for the charge rate. Since C ~ 1.5AH then 0.5C = 750maH and 50 to 100ma for the back light and device operation while charging wouldn't seem out of line. All of this aligns with a 850ma charger, so yes, a two hour charge from near 0% using the droid accessory would make sense. Likewise since a PC is limited to 500ma, the charge time will increase by about 40% so 3 hours makes sense here as well.
Charging a Lithium Ion battery at a substantially higher rate than the cell manufacturer recommends can result in reduced life or damage. Most decent lithium charger circuits are programmable and monitor/adjust the current going into the battery making an over sized external wall adapter useless. Lithium has the highest energy density of all the current battery technologies and gives the best run times, but when damaged it is also one of the most volatile. I'd just stay with the Droid accessories and be safe.
Note: "C" stands for the rated battery capacity