If you go into the IMU menu (Main controller settings > Sensors), in the IMU section, there is a bias value. I've noticed that the last two times when I've calibrated the IMU, this value was noticeably higher before the calibration than after, so my question is: Is this value an indicator about the necessity of having to calibrate the IMU, e.g. if it is very close to zero, there's no need to do it? And if it gets high after every two flights or so, is that an indication that there might be something wrong with it? Since some people seem to go for weeks without calibrating...

Thats right - the higher the bias the less accurate/precise it will be. As long as it is green/excellent then there is no need to calibrate. The only time I personally calibrate the IMU on any drone is after a FW update, if I experience drift whilst flying in light winds or the bias is amber. It's important that when you do the calibration that the drone is cool (at or below room temperature) and on a flat, level surface with no vibrations or disturbance to the drone. Always best to do the IMU cal. indoors with the props off.