Thousand-Year Reign

This is when The Empire, or some other organization, is somehow estimated to last a certain amount of time. 1,000 years is common, but there are likely other time frames. The important thing is that this is about assuming the length beforehand, not how long it actually lasts.

Film

 * In Gladiator, that is how long Commodus wants his incestuous line to last, when he thinks he's won completely.

Literature

 * The City of Ember was designed to last exactly 200 years and no longer. Too bad the note meant to inform the populace of this got lost.
 * In The Foundation books, Hari Seldon have several of these, where he anticipated how long each stage was going to last before the next Seldon Crisis.
 * In a curiously tasteless regional allusion, the German first edition was actually released under a title translating to "The thousand-year plan".
 * According to the 19th century pre-millennial interpretation of The Bible, after Armageddon, Jesus will reign over the earth for this amount of time before dad Jehovah steps in, which would make this Older Than Radio.
 * This takes place in Kingdom Come from the Left Behind series, during which only "glorifieds" and "naturals" who have put their faith in Jesus Christ will be allowed to live to the end of the Millenium, while "naturals" who stay unbelievers by the time they reach 100 years of age will die and go to Hell. The Other Light faction sees God Is Evil because of this and has prepared for this contingency by passing down their teaching to the next generation of its converts so that the generation that gets to confront God and Jesus at the end of the Millennium will be "assured victory" when Satan is released. Unfortunately for them, it didn't turn out as they hoped.
 * In Brandon Sanderson's Mistborn series, the Final Empire has said to have lasted for a thousand years. Interesting example of this in that it's been ruled by one man for the whole time, the immortal Lord Ruler. Toppling it - and him - is the goal of the first book.
 * The trope is mentioned in The Elenium, where a reference is made to a historical figure creating an empire of a thousand years that barely outlived its founder.

Real Life

 * 1,000 years was the prediction for how long Rome would reign made by Cicero.
 * He wasn't entirely wrong. The Eastern Roman Empire lasted until 1453, and the Ottoman Emperor claimed the title of "Kaysar-i-Rum" "Caesar of Rome" until 1922.
 * Guess how long Those Wacky Nazis expected the Third Reich to last.
 * Ancient Chinese emperors tend to be greeted with "May you reign for 10,000 years" but most dynasties only lasts about 400 years or so.
 * The ruling house of Japan is an inversion: the Japanese Tenno (emperors) claim an unbroken line that goes back more than 6,000 years, while the Japanese as a people have existed for less than 2,000 years, with no unbroken line to speak of (not even through adoption, thanks to lots of power struggles). The ancestoral line of Tennos was 'reconstructed' in the 7th century and its continuity is mostly fictional. However, it is fairly certain that the house itself has actually succeeded in ruling Japan for 1,000 years--twice. When those anniversaries occurred, though, is a matter of conjecture.
 * "Tenno hekka banzai" the battle cry that gives the phrase "banzai charge" means literally "Ten thousand years! [ to the Emperor]"
 * The Habsburg dynasty had in one way or the other held power for at least six centuries. But this hasn't stop some of the early ones from claiming descent from as far afield as Agamemnon and even King David from the Bible. Naturally, their descendants pretty much retracted those.
 * The Holy Roman Empire, in all its forms from Charlemagne to the Napoleonic Wars, lasted for just over 1,000 years (800 to 1806). This is part of the reason that Those Wacky Nazis, above, predicted the same for the Third Reich (by their parlance, the Empire was the First Reich).
 * The Imperial Germany, also known as the Second Reich in between the HRE and NG, lasted from 1871-1918. It might have lasted this long if it wasn't for losing World War One.

Tabletop Games

 * Vlaakith CLVII, Lich Queen of the Githyanki, has ruled her people for at least a thousand years by virtue of being undead in the Planescape setting of Dungeons and Dragons.

Video Games

 * Millennial Kingdom, as per Shin Megami Tensei

Web Comics

 * Irregular Webcomic: Nazi Science  sneers at your pathetic notions of arithmetic!

Western Animation
"Raimundo: Why is it always a thousand years?
 * The "Woodland Critter Christmas" episode of South Park, where the The Antichrist is supposed to "bring a thousand years of darkness to the forest".
 * Spoofed in Xiaolin Showdown. One episode revolves around them locking up their dragon Dojo for the day, because on that particular day he'll turn evil, grow two heads and devour all of the magical Shen Gong Wu items, leading to 1,000 years of darkness. The spoof comes from the quote below.

Master Fung: It's actually only 962 years, but a thousand sounds more ominous."