The duty cycle of the injectors is controlled solely by the ECU, based on information it receives from the various sensors on the engine (temp, air flow, throttle position, etc). The ECU is relying on the pump and regulator to be working "as designed" - it doesn't have any sensor input from them.
The ECU computes the injector opening time based on the info from the sensors, including how much air it has computed is available, and directs the injector to open for a certain time.
This is why engines melt pistons when fuel pumps start to "go off" - the pump can't supply enough pressure, which is the sole determinant of how much fuel gets injected while the injector is open. So the fuel mixture is over-lean (not enough fuel).