Somewhere between 1/32" and 1/64" - it's negligible.
EDIT: Well, maybe it's closer to 1/32" now that I think about it. It shouldn't matter though because you install the L-brackets/marquee retainer after making the sandwich so if you measure using just the thickness of the plexiglass theentire thing will stick out the thickness of the marquee which I hope wouldn't make that big of a difference whether it's 1/16" or smaller...