How much years of school do you need to become a therapist?

What kind of therapist?

You need a minimum of a master's degree in counseling, or a PhD/PsyD in clinical psychology. So 4-year bachelor's, plus 2 for a master's, more for doctorate. However, it can take you longer, for example, if you take reduced or part-time course loads.

If you want to be a counselor, then you can be licensed with a master's degree; that usually requires 4 years of undergraduate education for a bachelor's degree and then at least another 2 years for the master's degree.  

If you want to be a psychologist, then most states require you to have a doctorate in order to be licensed; there are very few that will allow you to be licensed with a master's degree.  The master's degree generally takes the same amount of time, although some of these programs are 3 years because they require a full-year internship.  Most doctoral programs that are training you to be a therapist are 5 years long - 4 years of classwork/schooling and then a year of internship.  That is all beyond your undergraduate degree.  

You can work as a social worker with just a bachelor's degree, but to do actual therapy as a social worker, then you will also need a master's degree in that.

Depends on what kind of therapist you are talking about, but I would suspect you would a least need a graduate degree of Master's, which would require an additional 2 or 3 years after getting your Bachelor's Degree in college (in the US).

You'll either need a Master's or Doctoral degree to become a therapist. First, you must receive your Bachelor's Degree which takes four (4) years, then you must obtain your Master's Degree which will take two (2) years so that equals six (6) years. If you want a Ph.D. degree, which will get you hired better than someone with a Masters, then you will need to work five (5) to seven (7) years to obtain your Ph.D. degree which equals eleven (11) to thirteen (13) years.  

I assume you mean a mental health therapist and not one of the many other therapists?  Its a masters--2-3 years after a bachelors.