This is always up to you but if I do half hour, I may tip $20 or $40 for full hour. If I don't want to see the girl again, I don't tip at all.
If I really want her to remember me and they often do, I could tip $40 for half hour or more on full hour so they know you value what they did for me.
It's up to you and everyone's different.
Some folks do not tip anymore because prices has gone up $20 to $40 and that ends up covering the tip that used to be given so they no longer tip.