The time elapsed is going to be approximately 20ms, but it will fluctuate depending on network latencies.
Just perform the calculation each time through the loop.
Code:
currentTime = Timer.getFPGATimestamp(); //time in seconds
elapsedTime = currentTime - lastTime;
//do your math
lastTime = currentTime;
More info on what the getFPGATimestamp method does here:
http://team2168.org/javadoc/edu/wpi/...PGATimestamp--