Any advice on visiting the Empire State Building?

My friends and I are planning a trip to NYC in a few weeks and we want to visit the observation deck in the Empire State Building. My understanding is that when you book tickets you have to select an arrival time, but I have also heard that sometimes there is also a long line to get in (we will be there on a Saturday afternoon so I expect this to be a popular time). So I am wondering, is the arrival time you book for your ticket the time that you are allowed to get in line? Or is that the time that you should plan to be at the front of the line and therefore you should actually get in line like an hour earlier than your booked arrival time? And would an hour be enough time to allow for the line? Any information or advice on this would be so helpful!