I did SRO for the Saturday evening performance on July 7. They actually didn't start selling standing until around 6:15p - the show hadn't yet gotten close enough to selling out when the box office opened at 10a. (As I mentioned elsewhere - I think on the TBITB rush thread - they said to come back at 4:30, then at 5, etc., until we formed a mini line around 6p. Keep in mind the week of the 4th is typically a light theater week, so our experience might not reflect other Saturdays... especially as we approach the closing.)
It's on TKTS today, and you can also look on telecharge to get an idea of how many seats are still available. They likely won't wait to 100% sell out before selling SRO, but I think they like to get pretty close... All this having been said, if I were going to try it again, I'd go when the box office opens and politely ask then if they're selling SRO - and not be surprised if they say to come back later.