I would say "never" since it's a Universal Studio's attraction.
It's due to open Memorial Day Weekend.
PS: Travel writer Simon Veness has done a terrific write up here on the DIS regarding the new attraction. Spoilers are included, but it's well worth a read if you're interested. Find it HERE
We were at USF 13th and 14th and it was open...I think I read it offically opened on like June 11th or around there...Shrek is a really good ride!!
the line got really long(In the morning it was 75 minutes!)but towards the afternoon it got down to 45...but we had FOTL so we didn't have to wait nearly that long.