It's not too bad, on the whole - but any tattoo hurts like the proverbial mofo! The back and shoulders are more painful than the upper arms, but it really depends on the individual. If you're having a bad day it'll hurt worse, bizarrely.

Plus of course, the average 'girly' tattoo takes about 20 minutes - the stuff I've had done takes hours at a time...!

But yeah, it depends. A quick way to check is to pinch the area; then compare it to pinching the area on the outside of the upper arm. That gives you a rough indicator.