[dubiousSwain]

Hmm...
I don't know much about C++, but I would start by triggering a timer function when the sensor reads black, then when the sensor reads white, write the timer function's time to a variable then use that in an elif statement (excuse my python)

something like:

void main() {
      if (sensor<black){
            startTimer();
      }
      if (sensor == white && timerStarted == true){
                if (timerValue<12600){
                        doSomething();
                }
                if (timerValue>12600){
                         doSomethingElse();
                }
         }
 }

PM me if you need anything else