It depends on so many variables! Let's try to make it simple.
For starters, I'm gonna assume that the two batteries are identical, standard deep cycle automotive lead-acid, 12V, 100Ah, brand new. One of them is fully charged and the other zeroed.
The empty battery will request almost all available current on firsts seconds, which in this case, near 1100A. (assuming a zeroed battery have a standard internal resistance of 5mohm. It can vary, and you can look for the battery datasheet). And it will kill the battery due to overheating. Not a good thing.
But if you have a current regulator, you can plug it between the batteries and set the current to the standard maximum charge current (which is about 25% of the battery capacity), in this case, 25A, and let them equalize. Using this much current, the losses will be around 30% to 40%. Then, after a few hours, you gonna have the two batteries equalized with something around 40Ah of charge each.