13.6v

That will get to where no more amps are going in, which is one notion of "full." But the joke's on you when you check with an hydrometer and see the SG is only "fair" instead of "good." (13.6 is below the "gassing voltage")

To recharge 6s fully, you need to do this:

http://pdf.wholesalesolar.com/battery-folder/charging_instruction_2011_2.pdf

Yes, 12v bank.

Having seen PD's 13.2, 13.6, 14.4 graph numerous times here, just wondering in 13.2 or 13.6 given enough time (days, weeks ...) can get a battery back to 100%

It will get there at that voltage. I am assuming that your 6V batteries are hooked together in series to give you 12 volts.