answer 1
Depends on how fast your hair grows. With this productg I notice roots show before the color of your hair fades/grays show thru. So since you only apply color to the new growth--just color roots when they show thru. Don't apply to whole head even for the recommended 10 minutes every time--maybe every other or third time depending on how fast the product fades on you. How fast the product fades depends on whether you are using color safe products and how porous you hair is.
answer 2
This answer really depends upon the condition of your hair and what color process you intend on using. The general rule is 6-8 weeks, however adhering to this will not deter your hair from breaking. The best motto is to color it as little as possible. Incorporating a few colors into your highlights help to blend in your natural color when your hair grows. This process allows you to avoid coloring it more much longer. A single or double process, however, shows roots almost immediately and makes it difficult to stay out out of the salon!!!