Usually desulfation is achieved by passing "high voltage" (often around 50V) through the battery. The simplest way to do this would probably be by using a 555 timer and a few other components to send the high voltage pulses into the battery. Generally desulfator circuits draw around 300mA, though it can fluctuate widely depending on the design you use. If it starts drawing >600mA then chances are you're doing it wrong.
Desulfation will usually take quite a while before the effects are completely reversed--some people have stated it can take up to a month, and possibly even more. You'll need to do some extensive testing on your circuit to make sure it's not drawing too much current and that it stays cool, so that it will be able to last that long. Ideally you would have it powered from a wall wart, so you don't have to worry about draining any other batteries. Frequency will need to be around 1KHz, if I am not mistaken.
So in short, the answers to your questions are as follows:
- You will need to be able to pulse around 50V to the battery, and have a power source capable of handling preferably >1A (even though the circuit should draw less than half that, it'd be a good idea to be able to supply more just in case).
- The charging will probably take a while. Once your battery can hold a charge of around 13.5V then it should be ok.
- Just assume around 1000 Hz, though you'll want to make it adjustable.
NOTE: I just found a quick n' dirty battery desulfator circuit on Instructables. I don't normally recommend following electronics circuits from there, but this one may be worth a try. Here is the link: Battery Desulfator.
The above project uses the following schematic:
I hope this helps, and I wish you the very best of luck! Please post back here with the results once you have them!