I would like to know if and how often you have to calibrate the MQ sensors.
If you want to measure the concentration of a specific gas, then you must calibrate the sensor in an environment with a known concentration of that gas, and preferably more than one concentration.
The sensor needs to be "burned in" for up to 48 hours before use and before calibration, and if it has been turned off for a long period of time, burned in again for a shorter period of time.
There is a lot of very useful info in this blog post on making an electronic nose using MQ sensors.