I'd aim for 30 minutes ahead for the perimeter security. Those with reserved slots use a different leg of the tower than those waiting in line for tickets and access. There are big 'reservation' signs where you need to go. No point in searching for a restroom etc on the ground before going up the tower there are rest rooms, snack bars, souvenir shops etc on the second floor of the Tower. You will need to line up again for elevators to the top and that is entirely first come first serve and can be a bit slow. Leave plenty of time for the whole experience. If the line is not too long for the top, go ahead and do that first and then come back down and enjoy the views and amenities on the second floor.
I'd want to be in the line for the elevators about 15 minutes ahead of my time slot or so.