Hey guys how often usually calibrate the phantom? I know that a few calibrate before every flight.

I would run through the calibrations once when new, and thereafter Only when prompted by the system, or perhaps if some erratic flying by the aircraft needs investigation.

No calibrations should be required before each flight.

My calibration list (for the initial calibrations) are:-

1) Sensors (Using DJI Assistant 2 v1.1.2-2, latest version), usually done just after update on Assistant 2.

2) IMU (level surface, make sure it's had time to cool down if it's been powered up, say half an hour at lowest ambient temperature, a cool evening or air conditioned room below 10 degrees C helps).  
Using the newer '3D' IMU calibration routine, my last two IMU calibrations have been done at about 20C ambient air temperature. The subsequent IMU warm up time when aircraft is first powered up and initialising does not seem to have been adversely affected, and warm up time is a few seconds at most.

3) Gimbal (just after IMU).

4) RC sticks (with aircraft off).

5) And compass (in an open area (not over steel reinforced concrete), away from ferrous materials or electromagnetic interference). Get one good calibration, and that should be all that is needed until informed by system or erratic flying that needs investigation. Note that any ferrous metals very close to legs (steel frame in desk, cutlery in kitchen drawer, windowsill strengthening etc) will generate a 'compass error', this does not mean compass needs recalibration. just move the aircraft away from source of interference.

Unless prompted, I never calibrate anything. The most warnings you will get is usually the compass.  All you have to do then is to move to a different area, usually only a few feet, and the warning goes away.

I do a IMU, gimbal, sensor  and compass update after any firmware update.  Some may say that is not necessary, but one of the things I do before I update the firmware is check all the settings pre-update AND post update.  
Twice ....once with my P4 on the last update and also with the last update of my Inspire.....the values/bars were way off after the update.....in fact the P4 had one of the accelerometer (ACC2) values way in the yellow after the firmware update.  Recalibrating the IMU took care of the discrepancies in both cases.
As far as the ....I only calibrate (other then firmware update) when I travel to a place more then 100 miles from my previous spot.  Some say no but its my preference.
One thing I can tell you is that I am religious about checking all the settings in the app before taking off.......any bad/off readings and I move positions.   

Once per year, or when there are hardware modifications or a major firmware upgrade, whichever comes first.

Apart from the original calibration, performed in the UK in April this year when new, and once more when back home here, I do not calibrate unless prompted (which has never happened. I do however recalibrate the gimbal from time to time if I think the horizon appears out of whack.  That invariably works.

Generally, a warning message will pop up if something goes wrong. Calibration is required when the message prompted. No worries.