I found this
http://www.1stweather.com/
Using the Metrocast dropdown I find that RH is reported at 83% for Joeys, 48% for CT.
AFAIK you should start humidifying when RH drops to 40%. Andy McGibbon told me that if your skin feels dry then it's probably time to start humidifying.
Bob Taylor in the
Acoustic Guitar Owner's Manual says that most production lines are conditioned to about 45% and that 30% is when you will usually start noticing a lowering action, buzzes and maybe the fret ends starting to protrude.