I can't quite recall, but I'd guess it may have been 45 minutes to an hour (my family loves to shop after closing at MouseGear). If you're near a CM with a park radio, they'll give the CMs a warning a couple of minutes in advance not to panic when they hear the "Boom" (they've since added a PA announcement). We've seen it a couple of times now. Basically they blow off all of the excess LPG left in the barge's tanks with one
big fireball. Here's a video of it:
YouTube Link.