The battery in a PA34 is a Gill G35 or G35S.
It has a 30 minute capacity rating of 40 amps. This means it will if fully charged deliver 40 amps for 30 minutes and the cell voltage will still be more than 1.75V per cell, whilst at 25 deg C.
Gill state that their batteries remain serviceable until they can only deliver 80% of that 30 minute demand.
So you have at the very minimum 16 amp/hours of supply - at the end of which your battery is still producing a significant level of power but the remaining ouput is very dependent on its condition.
If you load shed thoroughly your battery will last considerably longer than the fuel in your tanks. Continue using the pitot heat and all the lights 30 to 60 minutes.
And battery capacity was added to the LAMS schedule as an annual check - presumably because so many people think you cannot fly without electricity?