Hard to say, but I would guess as much as 1/16 with ribs 3 in apart. Maybe I will actually go measure. The droop is minimized by the usual practice of running the grain spanwise. I have seen in some old, pre WWII, freeflight stuff the advocating of putting the grain chordwise to increase droop between the ribs. No explanation, but at that time they were using fairly thick wings; RAF 32 airfoil was popular. Some designs used multispar wings to minimize the droop around the front of the airfoil, and, perhaps, do some turbulation. I did that on the first Twister I built, two 1/8 square spars, top and bottom, ahead of the main spar. As I recall, the airplane had very little glide.