//Button Press Duration
unsigned long buttonPressStart=0;
unsigned long buttonPressDuration=0;

void setup() {
pinMode(2, INPUT_PULLUP);
Serial.begin(9600);
}

void loop() {
  if(digitalRead(2)==LOW)
  {
    if(buttonPressStart==0)
    {
      buttonPressStart=millis();
    }
  }
  else
  if(buttonPressStart!=0)
  {
    buttonPressDuration=millis()-buttonPressStart;
    Serial.print("ButtonPress:");
    Serial.println(buttonPressDuration);
    buttonPressStart=0;
  }
}