Some schools will drop some of the students from the waitlist and will send them a letter/email informing them of such.
Others will just let the summer pass, and if school starts and you haven't been offered a spot, well then..
Some schools allow you to specify the last date by which you want to remain on the wait list (i.e., early summer, mid summer, right before school starts, etc.) based on the amount of time you'll need to find housing, financial aid, etc.
The school can wait until the day before school starts to notify you of an available spot, but that might not be feasible for each student.