It depends on the school. The online school I used to go to has classes that are twelve weeks long. the one I'm going to now the classes are only five weeks long. You can enroll when a new session is about to start. The best thing to do is contact the school you want and tell them when you would like to begin. They will help you start whenever the closest start date is to what you want.
it goes on al the time year round just depends on the school u r interested in