Well, if we look at the Earth and the stages it has been through, then you will see that the time of mammals dominating the world is still just beginning.
Reptiles lived for a rough 200 million years beofre being wiped out by the asteroide that struck the gulf of Mexico, and so far this era has been extremely short. If world history is anything to go bye, then we still have at least a couple of million years left until our race is wiped out, a new age begins and a new species takes our place.
As far as the entire planet ending, I agree that we have an uncalculated amount of time remaining. The most probable way of all life on our planet ending is the sun imploding, seeing as stars die out every few seconds. The sun will have to end one day or another, and even if it does end soon, it has been confirmed that light would continue reaching Earth for a further four years, giving us at least a little time to prepare ourselves.