Three US presidents have been impeached: Andrew Johnson in 1868, Bill Clinton in 1998, and Donald Trump in 2019.
Trump was impeached December 2019 by The House of Representatives, Article I of the Constitution states the House of Representatives "shall have the sole Power of Impeachment." What happens now? Well, Trump will not officially leave office, Impeachment is just a series of legal charges, it is similar to an indictment. The House of Representatives impeached Trump by passing articles of impeachment via a simple majority vote. Now The Senate will hold a trial regarding the articles of impeachment. The Chief Justice of the United States presides over Presidential impeachment trials. Two-thirds of the Senate must vote in favor of removing the president from office.
If a President is removed from office, The first few positions in the line of succession in the current administration are:
Vice President Mike Pence,
House Speaker Nancy Pelosi,
President pro tempore of the Senate Chuck Grassley,
Secretary of State Mike Pompeo
Secretary of the Treasury Steven Mnuchin.
There is no limit on the impeachment timeline, but political factors like elections and term limits affect the speed of the process.
More information on CNN News.