Compass and accelerometer re calibration

[Mick Dixon]

How often should I re calibrate the compass and accelerometer?

 

[Panagra1]

I now re calibrate once per session. Before I switch batteries, I turn off the propellers. But I leave the quad-copter and
transmitter on while replacing the battery. That seems to work. (Hopefully, there is no safety issue.) I could not find
any information in the manual about this issue.

I have also re calibrated after every battery change.