To become a dental hygienist, most places would want you to either have a bachelors or associates degree.
A bachelors degree takes 3 - 7 years and an associates takes 2 - 3 years and whatever one depends on you.
You would also need to have a license to become a dental hygienist. To obtain the license, you would need to take a test, which consists of 350 multiple choice questions. To pass, you would need to score a 75 or higher.
Depending on which route you take, it could be 3+ years to be a dental hygienist.