The pressure is determined by a pressure controlled switch that switches the pump motor on and off. For example, mine turns on at 30 PIS and turns off at 50 PSI. When the water pressure drops to 30 PSI, the pump begins pumping water into the storage tank, compressing the air in the top of the tank until it reaches 50 PSI and then turns off, patiently waiting unil I use more water and reduce the pressure to 30 PSI.
There are two adjustment screws on the switch for the upper and lower pressure settings.
NOTE: THE SWITCH USES 110V AND YOU CAN BE KILLED IF YOU STICK YOUR FINGERS (or for that matter, any other part of your anatomy) IN THERE.
How often the pump must run depends upon the size of the storage tank.
If you don’t feel comfortable messing, er, adjusting these factors, I’d call a pump guy.