Hi Scott and let me guess, the operating info that came with the IBS states that it charges the cranking battery first and then charges the auxiliary battery ( I haven’t read their literature ).
Most dual battery isolator manufacturers make this B/S statement but what actually happens is the isolator measures the voltage on it’s input terminal.
Once the motor is running, the voltage on the isolator’s input terminal will rise and once it reaches a set level, usually about 13.2v, the isolator will either switch on or it will start a short time delay, ( usually no longer than about a minute or two ) and the switch on.
Very, VERY few dual battery isolators have any idea of the state of charge of the cranking battery, they act on the voltage levels the alternator operates at.
So Scott, in your set up, you isolator should be switched on ( cut-in and connecting the cranking and auxiliary battery together ) within a few seconds to a minute or so after you start your motor.
But as to how long it takes to charge your cranking battery, it all depends on how high or low it is when you start your drive.
But on average, with a fully charged cranking battery when you started your motor, a 15 minute drive will replace most of what you used to start the motor.

