I went to my local Apple Store last year to get a S4 Watch. It was pretty late, maybe 10:30am and they had a decent stock of most things, lower stock of certain premium models. There was a guy outside checking stock before you go in but there wasn't really a queue, just people waiting for the next person to be available.
If you get there at 8am you should be able to get anything you want, a little later and you probably won't get a green one.
As for opening time; usually all stores will open at 8am. The one near me is in a shopping mall but they must pay them to open the doors early once a year or something, I think most are the same because I know of at least 2 cities in the UK where this happens.