How do I get an actuate 'stopwatch time'?

I have constructed a program where an LED turns on and a counter begins timing how long it takes you you hit the vibration sensor. I have to counter incrementing inside a while loop, in the void loop function. Just by estimating using an actual stop watch I have come up with increasing the counter by 0.000125 every loop. Is there any function I can use that returns how long each loop takes?

You could use "millis()" or "micros()"

Check the stopwatch class - http://www.arduino.cc/playground/Code/StopWatchClass