How much to tip a hair dresser?

my cousin is manager of the shop but i do have to pay for chemicals still. its going to be 45$ for the chemicals, 3 differant colors. but how much of a tip would u give her? it will take a few hours she said.How much to tip a hair dresser?
when i get my hair colored i usually tip about 15-20 depending on what i getHow much to tip a hair dresser?
5-10
$15
13$


Hope i could help!!!
do a twenty-percent tip.
10 or 15 dollars
about $10