How long should hair dye be kept for a perfect gray cover?

First time when I used a hair dye on my hair, my hair could cover my grays only moderately. My wife advised me to keep the color for a longer duration to get some dark color. However, when I tried pulling it for 12 minutes, it darkened my hair way too dark than the natural color of hair. I would like to know from some hair expert, how long should I keep hair dye on my hair to cover my grays perfectly. My natural hair color is ash brown; therefore, please advice aptly! Thanks.