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Battery capacity depends not only on the size and type of battery and its rated capacity, but it's age and condition. Capacity can vary GREATLY over time.
Figure that a battery should not be drawn down by more than about 50% of its capacity, so a NEW 100AH battery should not be expected to put out more than about 50AH before it reaches 50% capacity...about 12.2 volts...after which it should be fully charged.
You can figure total AH draw by multiplying the amperage load by the total time, e.g., 6 amps for 45 mins = 6 amps x .75 hours = 4.5AH.