There is exactly zero chance that FPs for GotG will last until 6:00 pm on a Saturday. I've never seen them last past 1:00 in the afternoon, and it's not unusual for them to be gone by 10:30 in the morning.
However, I don't now about a thirty-minute wait if the line doesn't go beyond the gates. We got in line at 9:30 one evening to a posted 45-minute wait, with the switchbacks not quite full. We got to the Collector's office in about 50 minutes, and the total experience took about 70. It depends on how much of the switchback area they're using, which sometimes is hard to gauge from outside: for us, we actually went into the area where normally people waiting to meet Groot line up.