That depends on how much debris the vacuum has picked up. If the iRobot has detected a full debris basket, it will return to the docking station, empty itself and then return to the area where it left off and resume the cleaning.
My iRobot will run for about 45 mins between three rooms before deciding to revisit the dock to empty itself. And if you're asking how it maps the floors, as the vacuum cleans, it creates a map that is sent to the app you download. From there, you can name the rooms and also tell the vacuum to stay away from a certain room if needed.
I hope I've answered your question.