It all depends on the person and their field of study. A Master's Degree is when you know most or all of the field. A Ph. D. is when you know all of it, and then some. With a Ph.D. you basically add more to the table. It takes a lot of studying and if you're not good with stress than I wouldn't suggest getting one. It can take as long as it takes honestly. But most people only take 2-6 years to get one.