Nobody knows because nobody has every come back from the other side to let us know, sketchy near-death experiences aside.

I have a few theories. You know how they say you see your life flash before your eyes as you're dying? Well, I think that your perception of time slows down and passes much more slowly for you than for everyone else. Who knows, you might be able to live a lifetime in the minutes that it takes your brain to die. So the next day while the living thinks that you are dead, and you are, your perception is that you're still going as you exponentially approach the moment of death.

Another theory is similar to the ending of Lost. Even though we will all die at different times, we will arrive in the afterlife at the same moment in time. Maybe trillions and trillions of years in the future when the universe is pure entropy and time has stopped.