Measurement of pulse duration greater than 3mins
The concept is actually quite simple.

You need to record the time when the signal goes high, and record the time when it goes low. The pulse width is the difference between the two times.

Yes, you can do it with interrupts, but you don't need to if you're not trying to do anything else at the same time.

A simple polling code could be something like this:

while (digitalRead(pin) == LOW); // Wait for input to go high
unsigned long startTime = millis(); // Record start time
while (digitalRead(pin) == HIGH); // Wait for input to go low
unsigned long endTime = millis(); // Record end time
unsigned long pulseDuration = endTime - startTime;


You basically sit doing nothing until the leading rising edge of your pulse.  You then record the time, and again sit doing nothing - this time while looking for the trailing falling edge of the pulse.  Then record the end time and subtract the two.