The time between every touch-up to cover up our greys depends on the percentage of grey hair you have. If your percentage of grey hair is more than 50%, then it is best to touch-up your hair within four to six weeks of applying hair color. It may seem expensive to some of the people, then you can touch-up your hair color at home. If your percentage of grey hair is less than 50%, then you need to touch-up your hair within eight to ten weeks.