Depends what the battery's capacity is, given in Amp-hours (Ah). This is how much current the battery will be able to supply without any recharge for 1 hour, e.g. 30Ah means the battery will supply 30 Amps of current for 1 hour. Or 15A for 2 hours, 10A for 3 etc.
If the alternator was to fail you would switch off all unnecissary electrical equipment in order to preserve the battery's charge so you can still work the radio, flaps or anything else vital.
Usually the Ah value is labelled on the battery though, is it not?
Smithy