I rushed during previews (which could be a game-changer in terms of rush popularity) but I arrived right at 10 - before the doors were open - and there were roughly 6 people ahead of me. Got tickets without problem, which were Left Orchestra Row P. It was for a Wednesday matinee, too. Also keep in mind that each individual can buy 2 tickets, so those 6 people could have bought 12 tickets.
I rushed like a month ago and got in line around 8am for a Sat matinee. The rush tickets were the last row of the mezz. It actually pissed me off because the majority of the mezz wasn't even sold. I moved up before the show started. Most of the other people moved at intermission.
"Pardon my prior Mcfee slip. I know how to spell her name. I just don't know how to type it." -Talulah
A friend rushed a two or three weeks ago on a Sunday - got there around 10am and a few other people showed up around 11, but that's it. Seats were in the middle of the rear mezz.