How much do you tip the shampoo girl in the hair salon?

I'm a guy with fairly short hair. I was wondering how much you tip the shampoo girl. I've heard from $1 to $5. I already tip my hairdresser 60%, so obviously I am not afraid to tip.





But that service is different than a 90 second shampoo. So how much for the shampoo?How much do you tip the shampoo girl in the hair salon?
Wow you tip 60%. You are sooooo cool.