I think it's pretty easy for UCLA prime to fill their seats so I'm sure their class is at 18 right now. When they drop to 17 is when they'll call the next in line.
So all we can really hope for is that someone who is currently holding a spot is accepted off of a waitlist to somewhere they'd rather attend, and that we're the next one in line.
My friend got a call a few days before may 15 and he turned it down that same moment, so I know they're actively calling when their class dips below 18.