What is the mAh of the battery? What is the mAh output of the charger? If you know the two, we can figure out how long it should take given the State of Charge at the start.
It will take approximately 1.25 times the battery's capacity in mAh to fully charge (to 100%) from fully discharged (0%). For example, the battery in the RAZR MAXX is 3,300mAh, and the charger puts out 750mAh. According to the manufacturer it should take 5.5 hours from fully discharged to fully charged, so 750 X 5.5 = 4125 / 3300 = 1.25. The same rule should apply for the typical battery as long as you are charging to 100%. For example, the Droid RAZR has a 1,780mAh battery and it uses the same charger, and the manufacturer says it will take 3 hours. In that example, 750 X 3 = 2250 / 1780 = 1.264, or about 1.25 times.
So reversing that calculation, if you have a charger that puts out 800mAh, and a battery that's 2500mAh, 2500 X 1.25 = 3125... That's the amount of total current spent to charge from 0% to 100%. 3125 / 800 = 3.9, so based on that calculation, it should take about 3.9 hours to fully charge a 2500mAh battery from 0% to 100% with an 800mAh charger.
Using a higher output charger may decrease the time it takes to get from 0% to about 90%, but will take longer to get from 90% to 100%, and overall will take about the same time. So using a rapid charger is only useful if you are NOT going to charge to 100%, but want a fast boost, let's say from 20% to 80% for instance. It will cut down the time getting from discharged to 90% of fully charged.
If the battery is starting with 20%, then reduce the factor of 1.25 by 20%, or 1, so then in the last example above, charging that 2500mAh battery from 20% to 100% should take 3.125 hours (2500 X 1 = 2500 / 800 = 3.125). This formula will begin to increase in error factor as you increase the State of Charge from the start, so starting at 50%, and charging to 100% should take LONGER than half of 3.9 hours, since the last 10% of the charge is done at a much slower rate and so takes longer, placing heavier weighting to the charge time.
Clarification; This calculation above also assumes you are charging with power OFF. With power ON, all calculations are completely arbitrary, since the device will use different amounts of power based on what is installed, what is running in the background, how much memory is used, if the display is on, what intensity the display is set to, if the WIFI or Bluetooth radios are on, if GPS is on, if it's on 3G or 4G, and countless other variables.
It is quite common to be using the car adapter, having the phone plugged into the adapter, running Navigation with GPS on, the display on, and with Bluetooth and 3G or 4G on and to have it actually DISCHARGE, rather than charge over time. The less the phone is consuming, the more power the charger has to put toward the battery. The phone's Parasitic Load will both impede the charging and screw up the metering circuits, possibly fooling the phone into thinking the battery is charged when it's still a way to go.
