There is no rule
of thumb that says charging for an hour will charge your batteries. Unless it is a smart phone
Your alternators, and I assume you mean 120A not 120amp/hr ?... You will rarely if ever see that sort of charge current
using an internal regulator
... perhaps 70 or 80A?.. (guessing) and even then the charge current
will reduce as the batteries accept charge.
I will often find it takes 3+hrs to charge to 100% from an overnight usage to 100%..and that is with a 100A alternator & 3 stage regulator
and overnight consumption
of 40AHr. so while i feel your 6hrs may be high, your consumption
is also significantly higher than mine and charging performance possibly less
You did not state if your alternators are controlled with an external 3 stage regulator, but if not, then you will see greatly improved charge performance if you add these... as the internal regulator of an alternator is set to approx 13.5V, whereas an external regulator follows a charge profile to improve efficiency.
if you do not have a clamp on ammeter and a battery monitor
with shunt they are good investments as you get real solid data on consumption and charging performance