the time of totality can be calculated from the size of the moon's shadow and from its speed and when you go to see a total eclipse you place yourself on the ground to intersect the track. Therefore your time in totality depends on how big the shadow is as it passes over you and about the maximum time you can get in totality is about 7 minutes (a 250 km shadow traveling at 1706 km/hour), but these eclipses are very rare and usually the duration of totality is much shorter.