I leave mine on probably around 45 minutes, depending on the hair color. If my hair was blonde and I wanna go back to a brunette then I leave it on longer..if i'm just dying it blonde when my hair is blonde to lighten it up i'll just use the minutes it says to on the instructions.
Definitely go with the instructions on the box. If you are dealing with a little gray then I would go with the longest amount that it says to do. Usually it will give you 2 different times to leave on hair so I would go the longest time for gray, long or thick hair. Hope this helps you & have fun with your new hair color! I love to color mine too!!