a good estimate would be 1 hour of charging per mAH on a 1000mah rate of charge, since galaxy s3 has a 2100mah, it would require only a little more than 2 hours of charging using a wall charger,
Sorry, but you are very wrong on many levels.
First, the phone charges at multiple rates. If it believes it is on a USB port, as some chargers indicate, It pulls about 300ma. max. If it believe it is on an AC charger, it pulls about 800ma max. So a full charge takes over 7 hours or over 2.5 hours. It depends.
Second, this is one of the smartest phones on the planet. It has magnetic sensors, G sensors, two cameras, voice band radios, LTE radios, WiFi, Bluetooth, GPS and Glonass. And you think it can't monitor the battery voltage? Wrong. It monitors the battery voltage and tapers the charge. It does not overcharge, and is smart enough to prevent an undercharge. When the battery reaches 100%, it shuts down. There is no reason to not let the SMART phone do this job for you.
Third, the battery pack itself has safety electronics in it. Look at the battery, it has a CE mark on it because it has to be tested due to the active electronics in it (I do this testing on occasion). This electronics in the battery itself prevents over charging and under discharging, plus it limits the current.
Fourth, you are comparing this battery to one used in hobbies like RC. I happen to fly electric RC helicopters and planes. Yes, charging those batteries requires a lot of care. RC batteries are raw batteries with no protection, not limiting electronics. The chargers I use are smart chargers. The batteries can be placed on charge and left for extended periods. I generally don't, simply because I use them immediately after they are charged. The load, planes, helicopters, etc, can easily over drain the battery, making the battery a hazard to recharge. Raw batteries like this should ALWAYS be handled with great care. The smartphones with removeable batteries have at least TWO levels of protection to prevent these issues. (note that a device with a none removeable battery may only have protection on the main board so may not be quite as safe). But in the end, you can not compare a raw hobby battery like RC, to a smartphone.
Fifth, the main reason the manuals tell you to unplug the charger when the battery is charged, is to save a fraction of a watt of electricity. Again, my lab also does Energy Star testing. Saving a fraction of a watt over long periods of time adds up. This is the goal they are chasing, and one of the things we test for.
Sixth, Samsung has sold over 20 MILLION SIIIs alone. There are hundreds of millions of LiIon powered smart phones out there. If just a tiny fraction, say 0.001% of them exploded and burned when left plugged in, there would be THOUSANDS of homes burned down by now. Someone would notice this. It hasn't happened yet (though Apple had a few poorly designed laptops that actually did about 6 years ago).
Sorry, but you missed the mark on this. Plug them in over night and don't worry. If you are that paranoid, you had better get a metal box and place it outdoors and only charge it outdoors sealed in the box. Me, my devices have been charging on the night stand beside me over night for about 15 years.