Rev 20:1-3
"Then I saw an angel coming down from heaven, holding in his hand the key to the bottomless pit and a great chain. And he seized the dragon, that ancient serpent, who is the devil and Satan, and bound him for a thousand years, and threw him into the pit, and shut it and sealed it over him, so that he might not deceive the nations any longer."
The millennium (a thousand years) is the most debated period in Christian eschatology. Three positions: premillennialism (Christ comes before the millennium), postmillennialism (Christ comes after), amillennialism (the millennium is symbolic, the current age). What is certain: Satan will be definitively defeated, Christ will reign, and the saints will participate in the kingdom.