the IMU rarely needs to be calibrated. If you notice the drone drifting, the IMU might need a recalibration. Using the gauges in the app only gives interference levels, not accuracy levels. I do make it a habit of recalibrating it about once every 6 months just because weather can affect the accuracy since it uses barometers and other weather sensitive instruments.
Compass should be calibrated more often, and I also recalibrate it about every 6 months. The compass should be recalibrated if you travel a long distance from the last calibration, since it works off the earths poles, its just a good practice to recalibrate if you travel.