From where will Jesus reign—from David's throne on earth or from heaven?

Scripture locates His future reign on earth, not in heaven. The government and peace without end come from the throne of David, and David's throne was on earth. That same throne is the one He will assume in Jerusalem, ruling with judgment, justice, and order. Thus His kingdom is earthly, centered in Jerusalem.

Les explains that the prophecy says His government and peace will have no end, and it is “not from heaven, but from... the throne of David.” He asks listeners to be logical: David's throne was on earth, not in heaven. That same throne is the one Jesus will assume in Jerusalem. The kingdom will be ordered and established with judgment, or rule, and with justice, and the zeal of the Lord of hosts will perform it because He is the Sovereign God.

Isaiah 9:7

Of the increase of his government and peace there shall be no end, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth even for ever. The zeal of the LORD of hosts will perform this.

How long will Christ rule on the throne of David?

Christ will rule upon the throne of David forever, not merely for a thousand years. His reign will continue right on into eternity under a new heaven and a new earth. The kingdom is established with judgment and justice from henceforth, extending beyond the thousand years into the eternal state.