Once a battery's been drained totally flat, my experience is that you'll either replace it immediately, by choice, or within two weeks or so, by necessity. If' the lights were left on, there may be a little life left; if you ran it too dry, it's a boat anchor now.
You need a way to measure the state of charge. A hydrometer is best, but can only be used on a non-sealed battery; a simple Radio Shack voltmeter works on all of them--if you don't have one, get one. With some drain on the battery (eg headlights) you should see at least 12.6 volts at full charge, about 12.2 at 50%, and if it's 12.0 or less you're flat. Under normal circumstances, your battery should never be as low as 50%.
I've never had a charging system fail, but I've replaced several batteries. A few have been run flat by error, but more often they've gone flat from lack of water. Use distilled if you can, reverse-osmosis "purified" otherwise. Avoid tap water except in dire emergency; it's better to drive a mile or two to a store than add the minerals and chlorine of tap water to the cells.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)