I don't know the exact rules, but it seems to be just whatever the staff feels. I've seen girls move from Hopeful to full S/G in just a few days, while other girls submit set after set and stay hopefuls forever. If there is any consistency, it is that the girls who move up fast have some exceptional quality that sets them apart from all the also-rans. Stick around this site long enough and you'll notice that most all the hopefuls pretty much look alike day in and day out. Check out Pixel on the front page. She submitted one average set back in 2009, but a couple of weeks ago she busted out with an AWESOME new set and jumped to full S/G in just a few days. That would be a good example and one I completely agreed with the staff on!
Doesn't need to be 100%. Around 96 is average I think. It's more to do with number of comments and general user consensus, mixed with the opinion of the staff, whether they feel it's appropriate/high enough quality for the site.
It's often a bit random though ... some great sets get overlooked then forgotten about for ages.
It's often a bit random though ... some great sets get overlooked then forgotten about for ages.