Too much!
My cat is a talker and she likes everyone to be up when she is. She also gets a little stressed when my husbands two girls come on the weekends, so after consulting with my Vet I decided to give this a go. The first week was really pretty good. I noticed some good change in her behaviours, but by the end of the second week I had no kitty left. She barely made a noise, extremely lethargic, almost seemed stoned. She's not playing at night anymore. Non of her other behaviours have changed though. She still goes outside. Still catches mice and still eats/drinks the same amount.
So yes, it works, but for my kitty. It works too good. My vet suggests I plug it in only on weekends when the girls are here so it's a lesser dose.