It becomes a simple math issue. First of all, take a look at your wall wart charger. It will show two separate outputs. One for 4.8 volts @ xx mah and the other for 9.6 volts @ xx mah.

The xx is the output current rating of the charger ports.

Just for the sake of calculating the time, let's just assume the RX charge port on the wall wart charger is 4.8 @ 100 mah (yours may be something else, this is just for information and calculations). Ok, then 1600 / 100 comes to 16 hours. Add about 10% for overhead and you will come up with somewhere around 17 to 18 hours of charge time. Definitely not overnight. The overhead, by the way, is heat and wasted, and there is not a thing that can be done about that. It's just there and you have to factor that into the charge time because heat is wattage (current X voltage).

This is, of course, if your battery is completely dry. If it is not dry, which it probably will not be, then the times will be less. The only way to really know is to discharge the batteries using a cycler and re-charge. This is if you want to know the real capacity of your batteries. If you just want to charge them, well, you will find out how much time it will take by just putting it on the charger and see how long it takes.

I'm not trying to be funny here, just that you really don't know exactly what is left in your battery. You can guess, if you know the load (close to precise load) that the batteries will be under during the flights, times the number of flights, minus the capacity.. then you will know what's left and how long it should take for your charger to recharge that battery pack. As you can see, it's not that easy to do. But, what you CAN do is put it on the charger for a specific period of time and charge it overnight. It should be pretty close unless it was completely drained.

Using something like a Hobbico Accu Charge.. the charge LED will blink when it's done charging, when it'd detected the peak charge voltage and switches over to trickle. Otherwise, using a wall wart charger, well, you will never really know, but as I said, you can pretty much guess that after x number of hours on a battery that has had x amount of discharge.... you get the picture.
CGr.