By far the longest. With that said there has been some app specific hiccups. Safari seems to occasionally hang, and it produces the same issue of no touch working in that app, but its specific to that app. I haven't had it happen in any other app so far. Before I was noticing it every single day I used the iPad in almost all apps. Still not sure I'd recommend going to a beta build for the improvement, but if that is your main gripe with iPad it is probably worth the update. Way more enjoyable to use now so far.
I do think they may have altered how touches are detected. Beta 1 broke palm rejection. In beta 2 that is fixed but I've noticed lighter touches/taps/glances that would have registered in the past do not register now. Not sure if they are trying to tweak their input detection to fix this or what. Wished they would give any insight as to what they actually change in their updates, kind of frustrating.