unsigned long dur_t, start_t;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(3, INPUT_PULLUP);
}

void loop() {
  // put your main code here, to run repeatedly:
  if(digitalRead(3)==LOW){
    dur_t = counting(3);
    Serial.println(dur_t);
  }
}

unsigned long counting(int pin){
  start_t = millis();
  while(digitalRead(pin) == LOW){
    //do nothing
  }
  return millis() - start_t;
}