Well, that's a tricky question...technically, Finnegan's and Duffs open at noonish or something. There's no reason to say you could go at opening and hang out the whole day. But they really start at park closing. Starting a little before park closing they'll put up some ropes/fences and will only allow you in if you have a valid HHN ticket for that night. If you are already in/at the place, they'll come around and scan your ticket or ask you to leave.
I don't think the park hours for Sept and Oct have officially been announced yet, but I think they Universal usually closes at 5pm on HHN nights.
Not sure what time they start the individual queue lines as I'm always hanging out in Finnegan's or at Duff Gardens.