I think the answer you are looking for can be found in the text of FAR 23.1353, which reads as follows:
[(h) In the event of a complete loss of the primary electrical power generating system, the battery must be capable of providing at least 30 minutes of electrical power to those loads that are essential to continued safe flight and landing. The 30 minute time period includes the time needed for the pilots to recognize the loss of generated power and take appropriate load shedding action.]
In real life, it might be longer, but don't count on it. If you follow the flight manual procedure for electrical system failure, and your battery is in good condition to begin with, it should not be less.
I hope that helps....