So I tried three times last week for the rush line, and here's what I learned:
They only guarantee 10 rush tickets per show. There's often more, but only 10 are actually set aside for the day-of rush line. They are in row H of the rear mezzanine to house left/stage right - literally the last row.
As for when to arrive, you need to be in line at minimum 3 hours before the box office opens to get a ticket. Ideally you should be there 4 hours early.
My first attempt was last Wednesday for a two show day; I got in line at 9am. The first 15 people all got tickets, and the rest of us got turned away. First in line arrived at 6am.
Attempt #2 was Saturday, again two shows. Got in line at 7:45am and I was #18 in line. They had one left but I needed two, so the person behind me took it. First in line arrived at 5:30am. 50 people in line when the box office opened.
Attempt #2 was Sunday. I arrived at 7:45am, but this time for a 12:00pm noon opening time, meaning it was over 4 hours in line. But this time I was second in line. First in line arrived at 7:30 (4.5 hours early). By 10am there was nearly 40 people in line.
I got two seats for $45 each, row H seats 103 and 105. The first six people in line got tickets, everyone else got turned away. They offered the few remaining premium seats at $179 each, and a few people took those.
Lessons: if you want the $45 Prima Facie tickets, it's a 4 hour wait to be safe. If you're past the first 7-8 people in line (per performance that day) you're probably out of luck and don't bother waiting.
It's a great play, and you can see everything clearly even from rear mezz row H. Worth the price.