Terms are 14-15 weeks long. Sometimes moving is optional, sometimes it isn’t. Some houses adopt a squatter’s rights policy where anyone in a room can choose to stay there regardless if someone with class seniority wants it. Other houses allow people with class seniority to choose whatever room they want, whether or not the current occupant wants to move. Sometimes, the type of room you’re in decides whether or not you get to stay. Some people with medical singles have to change houses. Sometimes, if you somehow have a double as a single, it doesn’t always stay a single and housing will make you move. It really depends.
But if you mean is it optional to move all your stuff out every term, nope. It’s not optional, even if you stay in the same room. During the summer, grad students live in some of the houses, and over field work term, students staying on campus are all condensed into a few houses.