There is no hard and fast rule on how long it takes for an individual to become dependent on drugs or develop a drug addiction. The length of time can depend on the type of drug being used, the amount taken, and whether a combination of drugs (including alcohol) is used. Other factors, such as physical and psychological health, can also influence drug dependence. However, drug dependence does not often happen immediately. It often takes weeks of heavy, chronic use before a person becomes physically dependent on a drug.