They sell rush two hours before curtain. To be safe, I would suggest arriving no later than noon, but if you feel like you want to line up, I don't think you would need to be there before 11 or 11:30 (or whatever the respective times would be for the performance you're going to).
I went on a Thursday night during the last week of January and got two tickets front row. We got there 5-10 minutes before 6 and there were maybe 5 people in front of us...