The amp per hour charge rate is dependant on a number of factors for example the length of time the engine/alternator are running and how quickly they run.
Most alternators are designed to trickle charge a battery so the longer and faster they run the higher the charge rate. (Up to the maximum capacity of the alternator and battery) There is also a restriction based on the maximum capacity of the alternator and battery in any system plus any drop off through the wiring due to resistance.
Batteries/alternator
At start up the engine requires a high amount of amps to start. How much will depend on the Vehicle and other factors. ( this can vary depending on temperature and weather conditions) Starting the car in low temp or below freezing will draw much higher current from the vehicle battery.
Once the engine is running the alternator is solely responsible for the electrical needs of the vehicle plus has to recharge the batteries.
On a modern vehicle this can include
Running the engine
Powering any engine management systems
Lights
Radio
Sensors
Wind screen wipers
Air Con if fitted
Heating system
On top of the above we are now charging a second battery from the alternator plus any device EG mobile phone MP3 player that is plugged into the Power outlet (cigarette lighter socket)
Both the Vehicle and leisure batteries are designed trickle charge, This means only low current is passed to the batteries as they recharge. It can take the Vehicle battery some time to recover amps used at engine start up.
The alternator will prioritise the vehicle battery and vehicle system over any auxiliary charging, This means only a small amount of the available current is used to charge the leisure battery at any time.
How much current is replaced in the Leisure battery is difficult to answer, An example would be the pumps draw around 6amp an hour. Now if we work on the basis that for up to 30 minutes the alternator will prioritise the vehicle battery and engine systems then current to the leisure battery may only be 2 - 3 amps an hour, less then is being used.
In honesty the above figure may be higher or lower, without testing the amps going to the leisure battery It is difficult to be precise.
If you are doing low milage and not running the engine long you will likely need to do a supplementary charge a couple times a week.
Also remember to Fuse both positive lines with the correct rated fuse,