A 100% dead battery will probably not recover. The people that complain about having a battery that is week or will not hold a charge have the batteries that can usually be recovered. Also if a battery has been let to go flat over many months will usually recover.
How well do they recover? well it depends upon the condition of the old battery. Some recover and perform as good as new, some recover but at reduced capacity but are still serviceable and some are just too far gone to bring back.
How long does it take? A long time. You need to cycle the battery several times to bring it back as much as you can, maybe 8 or ten times. At a C20 charge and discharge rate (see the earlier posts) that is 40 hours per cycle. No-one is happy to wait weeks to get there battery back so you have to have a stock of recovered batteries to exchange.
This is a little off topic but I will answer anyway. I know a few motor mechanics and once they knew about what I could do the word got around. A car battery here costs about two to three weeks wage for an average worker. I do not advertise but recover batteries as a service to a few garages. I do not charge much because this is a poor country and I only try to fund my research. The mechanics offer what they think it is worth. What is important to me is to learn what I can about batteries and how to recover them. I have a cheap Cold Cranking Amp meter and use that to give the mechanics an idea of the condition of the battery. When the word gets out people will be looking for you to help them.
To make the charging of larger batteries possible see how-to-increase-the-output-of-a-bedini-fan-t51.html
Cold cranking amps is a good guide to how well the battery will perform when on a vehicle but does not give you the full picture of reserve capacity
see Battery FAQs