I just did Legally Blonde rush on Saturday.
I got there at 6:15ish and was 8th in line and got 2nd row for the matinee. Some people in front of me got there, god knows when. But I say your safe bet is 6:30-7:00.
Of course, Saturday could be much harder than Sunday.
Good luck.
"Anybody that goes to the theater, I think we’re all misfits, so we ended up on stage or in the audience.” --- Patti LuPone.