It takes a long time, and letting the battery discharge completely can
allow sulfation to begin with means the plates are less likely to
accept a full charge.
Supposedly you can calculate "amp hours" against the cold cranking
capacity of the battery, but I would just check it periodically with a
hydrometer. You can get the really inexpensive kind with floating
colored plastic pellets, or quite a nice, accurate, temperature
compensated hydrometer from EZ Red for around $20

When I let one go completely dead, it takes 12 -18 hours, usually, at
6-10 amperes.

At 2 amperes, it will take, more or less, proportionately longer.

This is very rough, but may help you.

Turn your charger up to the max, which you say is 10 amps. If you have
a charger with a meter you might notice that the needle goes above 10
as it starts to charge and tapers off as it continues to charge. At
full charge it tapers off to almost nothing, less than an amp in many
cases. You need an independent means of testing the charge. You could
measure the voltage and stop when it gets to 14-15V with the charger
still on. Or you could disconnect the charger, drain the top charge off
the battery by turning the lights on for 30 seconds, turning the lights
off, and then checking the battery voltage. It should be about 12.6V.
In a case like this with a suspected bad battery, various things can
happen. It might sit on the charger forever and not come up to the
standard I proposeed above. The charge rate might never come up. The
voltage may never come up. It could appear to be fully charged and then
when you hit it with a big load like cranking the car, the capacity
might not be there. So be aware and don't run off and start looking for
other car repairs until you make sure the battery is up to the job.