It depends on whether you will be planning your timetable/degree before you go into uni? If you have everything sorted, the day can be over quite quickly, just a matter of getting someone to look over your stuff, sign it, talk about key points - done. But if you go in and have little idea about what you want to do or which units you want to pick, then the day can drag on....
Good time to check out the campus before the crowds come in.