I've done standing room several times, if you're planning on doing a weekday, you should be fine getting to standing room line somewhere between three to four hours prior to the time of tickets being given out. On weekends, the matinee line begins to start forming somewhere around nine-ish, and they begin selling the tickets an hour before the show.
I've done standing room for a Saturday matinee, got there around nine thirty-ten o'clock and was fourth in line. Also, when you're in standing room, you're given the option to enter the lotto, as well.
"Anybody that goes to the theater, I think we’re all misfits, so we ended up on stage or in the audience.” --- Patti LuPone.