Measure cycle count and time of Digital Input
You don't need to do anything on HIGH to LOW. For example, if you are counting how often a door opens you don't also need to count how often it closes, because it must close before it re-opens.

I don't see the point of the interrupt since your code is doing nothing else than checking if the interrupt occurred, so it may as well be checking if the input has changed anyway. This code is simpler:

const int MoldInput = 2;

int lastMoldState = 0;
unsigned long startTime = 0;
unsigned long TotalTime = 0;
unsigned long CycleTimeCounter = 0;

void setup() {
  pinMode (MoldInput, INPUT);
  Serial.begin(115200);
}

unsigned long lastReportedCycleCount = 0;

void loop() {
  int MoldState = digitalRead(MoldInput);
  if (MoldState != lastMoldState) {
    if (MoldState == HIGH) {
      CycleTimeCounter++;  // count times it was high
      TotalTime += millis() - startTime;  // add up time interval
      startTime = millis ();
      delay (100);    // debounce
    }  // end of state being HIGH
    lastMoldState = MoldState;
  }   // end of state change

  if (CycleTimeCounter % 10 == 0 &amp;&amp; CycleTimeCounter != lastReportedCycleCount)
    {
    Serial.print (F("Average time after "));
    Serial.print (CycleTimeCounter);
    Serial.print (F(" cycles is "));
    Serial.print (TotalTime / CycleTimeCounter);
    Serial.println (F(" milliseconds"));
    lastReportedCycleCount = CycleTimeCounter;
    }
} // end of loop