I calibrate my compass before every new flight.  How often and when should the IMU be calibrated.  I heard of someone's Inspire 1 flying away and DJI-Ken said you should calibrate the IMU before each flight I believe.  What is required?

IMU after every firmware update (although not strictly necessary it is good practice) or if the aircraft has been subject to a sharp physical shock.

Compass calibration once and just check mod values before every take off.

Totally pointless and run the risk of a bad calibration to do it every flight/

Tharg is correct.  Personally, I do an IMU calibration after each  firmware update but as far as the compass calibration, I wouldn't do it every flight.  As long as the compass readings are between 1,400-1,600 you're good to go.  

Pretty much what they said OP, however I would suggest that you also calibrate your RCs after a firmware update as well.
When and where to do a comapss cal is subject of a bit of argument at times, I usually calibrate compass when flying from a new location that is a reasonable distance from the last one.

A check of your sensor values before takeoff is sound advice.

You don't have to calibrate it every flight. but IMU calibration after every fw upgrade is highly recommended

DJI-Tim Posted at 2016-4-14 11:14
You don't have to calibrate it every flight. but IMU calibration after every fw upgrade is highly re ...

Before every Flight I "check IMU" in the MC-Settings as part of my checklist.
It takes only a few seconds. The IMU and the values for the sensors are very important.
What I have seen I believe that DJI wants you to check it before take-off if you want the DJI-Care.

dr. D Posted at 2016-4-14 12:23
Before every Flight I "check IMU" in the MC-Settings as part of my checklist.
It takes only a few  ...

Agreed, it doesn't hurt to check IMU and compass values. I also agree that you should perform an IMU calibration after every update.

dr. D Posted at 2016-4-14 12:23
Before every Flight I "check IMU" in the MC-Settings as part of my checklist.
It takes only a few  ...

I agree with you and this is on my check list as well!
I wish they would put the IMU on the top menu to make this eay to check. I know that it is not hard, but you need to go into the software three pages deep. Even more with the new P4...

Again, I agree with you and DJI Tim as far as the settings and only reset the IMU after the new FW is installed.

Rich

I always forget where IMU is and takes me 5+ min to find it.