Nothing can ever really 'end'. For something to completely end, that would directly contradict with common scientific law: namely, the law of the conservation of mass. This law states that matter can not be created or destroyed, only modified. Therefore, everything that exists today has always existed as either matter or energy, and everything that currently exists will always exist as matter or energy.
This being said, the matter which makes up Earth will never really 'end', but I suppose if you define 'end' as the annihilation of all life on Earth, then I suppose nobody really knows. One can make an educated guess as to when (And I'm not saying anything about that 2012 bullshit) such as when the Sun runs out of hydrogen to burn, but to repeat myself: Nobody knows.