Well it depends mostly on two things:
(1) How far down are the batteries compared to the size of your alternator
. If they are way far down and the alternator
is small (relatively) then it will be pumping away at max amps but not (yet) able to bring the voltage up to the 'bulk charge' voltage. But if the batteries are not so low and the alternator is big, the voltage should come up to the bulk charge voltage setting quite quickly.
(2) What the bulk charge voltage setting is should depend on your battery chemistry. Perhaps somewhere between 14.4-14.8 for deep cycle wet cells and 14.2 for gel cells.