Last year it was after 6:30...around 6:40ish? We got in line to see Flynn/Rapunzel et al one night and Cindy/Prince Charming et al the other at 6:40ish, and were in the first group to meet both.
Last year at the two parties we attended, they had a CM standing outside telling people that they weren't forming a line yet if they tried to form a line prior to 6:35 or so. After that, they then reconfigured the ropes and let people start lining up outside only, leaving the inside queue available for day guests right up until the party started (day guests who just wanted to see the princesses were ushered right into PFH). PFH is open for day guests right until 7:00, so they do need the space in the queue, and the walkway outside runs right in front of shops and restaurants, and at least last year was pretty narrow because 7DMT construction walls were still up. So they really couldn't have people lining up too early.
The line for the Seven Dwarfs formed much earlier, I guess because there was space for it and they had ropes set up. I thought they did a good job last year with the way they handled the princess/prince M&G. It was much more orderly than the year before when they were still meeting in Town Square Theatre.