Why a 1000 years?

Traditionally, amils–following the lead of Augustine–identified John's millennium (Rev 20:1-6) with the church age. However, that's fallen out of favor. Amils like Beale and Poythress now identify the millennium with the intermediate state.

Assuming this identification is correct (which, of course, premils deny), why would John use a 1000-year interval to designate the intermediate state? Although you have a few dissidents, most scholars agree that persecution and the impending threat of martyrdom is a major theme in Revelation. Indeed, amils interpret 20:1-6 in light of 6:9:11. The point is not that the saints in 20:1-6 are confined to Christian martyrs, but that martyrs represent the faithful. (Incidentally, this is quite pertinent to contemporary Christians in the Muslim world.)

A martyr is a Christian whose life is cut short when he is murdered (by a mob) or executed (by the state) for his faith. On the one hand, the 1000-year afterlife stands in contrast to his untimely death. Although he died prematurely, his afterlife in the intermediate state is far longer than if he had a normal lifespan, or even exceptional longevity.

On the other hand, the intermediate state is still a temporary condition. It's not the final state. It's not eternal. For the saints, eternal life is about more than the afterlife, more than immortality: it's about the resurrection of the body. Restoration of our physical existence. And that's illustrated by the new Eden, New Jerusalem denouement in the last chapters of Revelation. 

The duration of the intermediate state is more than this life has to offer, but less than the resurrection of the just–which is forever. And, of course, there's a qualitative as well as quantitative distinction. Reigning with Christ in heaven is better than life in a fallen world.