I had a local for surgery to remove a benign tumor on my hand. They pumped my arm full of it, so I was feeling no pain there. They also were giving me nitrous oxide, which I suppose was to keep me relaxed. The result was that I was very chatty, and found everything hilarious- even stuff that would probably have been disturbing otherwise.
As far as it wearing off, from what I recall, what you first start to feel when it does is that sort of pins & needles feeling, like when some bit of you falls asleep. The nurses and everyone pay close attention, and frequently asked how I was doing and all that. I guess if you do start to feel any change, just let them know right away, and they'll give you more. There's really no reason you should end up in real pain.
Good luck!