I was surprised last night at a practice to find my P22 9-volt battery was virtually dead ... I was wondering what the expected battery life (hours) should be?

When is the battery in use? I know it is in use when the Piezo output on the guitar is plugged in ... but is it also in use when only the magnetic output is in use? I guess it could be in order to have the mix function ... if so, perhaps that explains why my battery went dead so quickly ... I'm so used to leaving my guitars connected at home when I'm practicing everyday ... I'll have to get in the habit of disconnecting to preserve the battery ...