Why does my imu come up every time I switch the craft on the DJI Mavic Pro I have refreshed the firmware. The imu morning sometimes stays on 3 to 5 minutes before it goes off does this happen to everyone else drone I have calibrated it so many times

You don't need to calibrate it, then IMU needs to warm up before aircraft will start it's the same for everyone.
You only ever need to calibrate when you are prompted, I will post an article I wrote might give you a better understanding of what IMU is.

IMU "Inertial measurement unit" .
This was not written by me , but was an answer to a question I posed.
As far as I see it, IMU calibration on a level surface updates a table of values the flight controller software uses as a reference for a level stationary hover. From there the craft responds predictably to flight commands.  It also likely measures any sensor noise and thermal drift so that these technical imperfections are accounted for when using IMU sensor data in flight. - this is likely why there must be no vibrations during the calibration process.


Bad IMU calibration could cause drift and attitude issues as the flight controller fights to hold the craft in what it thinks is the correct attitude as opposed to the correct physical attitude.


The IMU usually has 2 types of sensors – angle and acceleration and in turn 3 sensors of each type measuring in the X,Y and Z axis. These sensors can, through vibration, aging, impact etc, drift in their response over time and thus an IMU calibration will establish a new reference for the IMU’s level/stationary state that the flight controller can work with to restore stable flight.


Think of calibration as the bringing back into line the measured craft attitude with the true physical craft attitude.


As to how often an IMU calibration is needed, Refer to your manual ; certainly after any impact or if there are suspicions the craft can’t hover in a level attitude in a windless environment.