unsigned long startTime;
unsigned long endTime;

void setup() {
  // put your setup code here, to run once:
  pinMode(10, INPUT);
  Serial.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly:
  while(digitalRead(12) == LOW);
  startTime = millis();
  while(digitalRead(12) == HIGH);
  endTime = millis() - startTime;  
  Serial.print("I received impuls with length in ms : ");
  Serial.println(endTime, DEC);
}