Understanding IMU Calibration and Allan Variance calculation

[RajS999]

I was referring to calculation of gyro and accel white noise and random walk using the video Visual-Inertial Sensor Calibration - A Complete Tutorial and Discussion for my Intel Realsense. I have following doubts:

1. At timestamp 25:48, video says, typically we dont use values generated by Allan Variance ROS directly and we inflate them by order of 10 times, may be by 20. Why is it so?
2. I read we can obtain Angle Random Walk, Bias Instability, and Rate Random Walk from Allan Variance plot by reading Allan Variance value at T=1, Slope=0, T=3 respectively. Why exactly Angle Random Walk is read at T=1 and Rate Random Walk at T=3 and not at some other values? Is it that these types of noise are "empirically observed" to most dominant in Allan Variance plot at these cluster interval values?
3. How often and when we have to calibrate IMU and calculate gyro and accel white noise and random walk (if we are flying drone in different temperature environment)? Do we have to calculate gyro and accel white noise and random walk every time we calibrate the IMU?