In my opinion, rolling is one of the most important things of parkour to learn. It has the potential to save your life. Someone who has practiced rolling for a couple years is likely able to roll 10-15 feet onto concrete, or 30-35 feet onto grass, but it depends greatly on the individual. The physics behind rolling is extremely simple too, it all has to do with Newton's second law.

    Every time a traceur comes in contact with the ground, there are two main forces acting upon their body. Those are the downward force of gravity, and the upward force of the ground. Remember that Force = Mass * Acceleration. You have no control over gravity or your mass as you are falling, meaning that you have to control your acceleration as much as possible, and it all has to do with how fast something stops moving. A perfect example is catching an egg. If you try to catch one but keep your hands still, it will break. If however, as it falls into your hands, you move your hands with it, it gives it a lot more time to slow down and won't break.

Image from http://nerdist.com/ocarina-of-time-taught-us-about-parkour-physics-before-it-was-cool/

    By increasing the time it takes you to stop accelerating, you're decreasing the force on your body. Lets say someone jumps out of a 3 meter tall window. Their gravitational potential energy (Mgh) is going to be converted into kinetic energy as they land, meaning that:

(The three following formula images were created by me)

    If we assume the person weighs 70kg, or about 155lbs, We can calculate the force on the body as they hit the ground.

    The force on the body depends on the time it takes to slow down. If the person locked their knees and had an impact time of around 0.01 seconds, their force would be around 55,000 N. However, by rolling, you can increase the impact time to around 0.4 seconds. This would make the force on you to be around 2,000 N. It is still a significant number, but if done right, you can even leave the roll running.

(Example found at http://www.popsci.com/entertainment-amp-gaming/article/2009-05/physics-free-running)

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“When you jump off a high cliff, if you hold forward, you will roll on the ground when you land and won’t get hurt from the fall. I can’t guarantee it will work, though, if the cliff is really, really high, heh heh!” -The Legend of Zelda Ocarina of Time Deku Scrub.

Image from http://nerdist.com/ocarina-of-time-taught-us-about-parkour-physics-before-it-was-cool/