Depends on the school. For example at Duke you need to meet with the dean of your dept and get approved for leave, then when you’re ready to return you need to show them what you’ve been doing in your absence to get approval to return. At UCLA IIRC there’s no real process, you just don’t register for classes the next quarter, and when you’re ready to return you just sign up again. So quite a big gap there, with probably a lot of variations in-between.