The math is a good estimate, but the best test is just running it. Just set everything up in the house... mics, minime, recorder running normally. Run the minime on battery, the recorder on AC. Record the random house sounds, set the gain to whatever it is at a typical show.. Let it run until the battery goes dead, overnight if necessary. The next day look how long it recorded. Depending on the recorder, the clock will stop at that point, or it will start recording dead air. Either way, you can figure out out. Different mics use different amounts of phantom power, so do the test using the mics you will run at the festival. On the other hand running 16/44 vs 24/96 shouldn't matter much, and recording loud music versus a quiet house shouldn't matter as far as battery life is concerned.
Then subtract 1 hour for safety.