I use 0% on planes that I'm used to. On new planes I will only use it on medium rates(high rates with minimal expo). One guy at the flying field was using somewhere around 80. So there isn't any set "right" amount. I would, however, start with 30% if you've never flown expo before and want to try it. I used to use expo a lot more, but after 7 years of flying without it, I coudn't fly well. On G3, I would try to fly rolling circles on the 3D Yak that had 80% on aileron and rudder, and I couldn't fly it. I took all expo off and could fly it easily.
Woops