Off hand I would say it is fairly thick, and by that I mean thick enough to weld to. The swingarm has to be quite strong, and as the driveshaft side is likely not as rigid as the the other side of the arm, I'd bet it is thicker than any other place on the frame. But without drilling a hole in it and measuring I have no idea of the exact dimension.
reason i ask, i want to tack weld a couple small brackets to the swingarm and perhaps (although im not 100% on this) the driveshaft cover to mount my fender to the arm.
i think to be on the safeside i may use the bolts that exist on the driveshaft cover in leu of tackwelding on a bracket but we shall see...
Near as I can tell from the spare swingarm I've got, the right side and the tube are about 3/32" thick; it's more than 1/16th, but definately less than 1/8". Don't take that as gospel; I'm still on my first pot of coffee, and don't have a micrometer. As KM said, without drilling a hole, or without some kind of measuring device that'll reach far enuff in and give an accurate measurement, it's a guess-by-how-far-my-fingers-are-apart kinda thing.