First off I've been going to my girl for, gosh, 7 years?! I absolutely love her and would never even dream of going anywhere else.
Anyways, when I go in to see her it is usually for a trim and high-lights. Also, I do have a lot of hair, it is extremely thick.
It ends up costing my a little over $100 before tip. I tip around 20%.
I think it depend on your relationship with your stylist. If you just go to random people then I'd probably only do 15%. But when you've been seeing someone for a long time you want to treat them well, cause they'll do the same.
IMHO it is better to over tip than under tip! (I am also an ex-server)