It'll vary based on the line on any given night, so I don't think you'll find the answer.
Last week I got in line at 6:05, was out by 6:45 and met Anna & Elsa. They were still listed on the sign as being available, and the line moved quickly (probably because there was no FP+).
By the time I left, they had removed Anna & Elsa's photo from the sign and were telling Guests it would be a visiting princess.
There have been reports of meeting the girls up to 7:30-45ish.