You should talk to your dealer where you bought your Prius. I don't have
a Prius yet, but on my current car, a friend's two young boys quickly
learned that they could trigger the alarm on my car fairly easily by
banging on it slightly. I asked the dealer where I bought my car if the
car would be less sensitive and he said it was a minor adjustment.
Either way, my friend's kids are past the age now where they are amused
my triggering my car's alarm system.

As for how long the car's alarm goes off, just take it to a park or
somewhere where the alarm won't bother anyone. Trigger the alarm, and
see for yourself how long it blasts away before it silences.