How many days does it take to get tooth decay?

This is different person to person. Tooth decay relies on many factors including the foods you eat, how often you brush and floss, and the pH of your saliva. The lower the pH the more likely you are do get decay. Some people are more prone to decay than others. It does not take long to get tooth decay if the environment is favorable to bacteria. I would say the quickest it could happen is one week.