Microscope is the game that never ends...
...it just goes on and on my friends.
Microscope is a collaborative world building game, which allows you to jump around time periods, fill them out as you see fit, and then bounce around to other times to ask questions and flesh out the history more.
On each player's turn, they can add a time period, add an event to that time period, or add a scene to an event.
Because every player is adding something each turn, the game could literally go forever.
How long does stuff take?
Setup? Generally 30-45 minutes depending on how many times around you go with the must/must-not list.
Adding Periods and Events take very little time, at most a few minutes while the player decides what they want to add. Scenes, however, require the players to act out the scene until the question of that scene is answered. This can vary wildly. I've literally seen a scene end in 30 seconds, while some of our more robust scenes took 15 minutes or longer to complete.
Setting Time limits
Ideally, what you'll want to do is figure out a number of "rounds" (round trips around the table), that you'll want to play before calling it quits. Every time a round completes, a new player becomes the "lens" (the person who determines the "theme" of the round). You'll probably want each player to get time to do that once, so in a 4 player game - 4 rounds minimum. The time a single round takes really depends on how many scenes you play. If no one makes a scene a single round can go by very very quickly. Otherwise, I'd say allow at least 30 minutes for a round, and if a scene is dragging on try to wrap it up by answering the question.
Continuing where you left off
If you want to "save" a microscope game, you can grab the index cards, place them in order, and then stash them away somewhere so you can play again later.