I think even a billion years for the existence of the human race is a stretch. Zombies will own the world by that point!

Seriously though, we, as a race, likely will not exist long enough to be present for the end of the planet. Continuing on our current course, we may be lucky to get a couple of million years. By then, I would ASSUME we would have the capacity to travel off-planet, perhaps doing terraforming & the like. But, as a race, going the direction we currently are, I dont see us having to worry too much about a natural world-ending event-unless natural is defined as human-driven...