How often do you have to change your altimiter?

As you progress during you flight, ATC will give you updated altimeter settings below 18,000 ft. You are required to use an altimeter setting for a reporting station within 100 nm. of your location. Leave it set to what ATC tell you. The rate of change depends on pressure systems along you route of flight. If unable to receive/acquire a altimeter sitting, such as take off from a remote uncontrolled airport, use field elevation for take off and update setting as soon as able.

Also, make sure you tune into ATIS on the ground, that has the vital information including pressure. Then you let ATC know you heard the ATIS transmission by saying you have information alpha/beta... etc. It changes a bit more with real world weather because the weather updates itself every 10 or 15 minutes, often the changes are quite drastic, so that will cause the pressure to change. 😉