According to For Dummies, Catholicism teaches that a person becomes a saint at the instant he enters heaven. Canonization is the process by which the Catholic Church recognizes certain people as saints. Several steps must be followed before canonization can take place, and the process normally does not begin until five years after a person has died.
How Stuff Works points out that it may take decades or hundreds of years before the Catholic Church officially recognizes someone as a saint.
Before the canonization process occurs, a pastor submits the proposed saint's name to a local bishop. The bishop analyzes the proposed saint's writings and life for proof to support the case, and the information is forwarded to the Vatican. A committee must approve the proposal for it to continue, and the pope must declare the person to be venerable. Next, evidence must suggest that the proposed saint caused two miracles after his death. However, martyrs do not need the miracles stipulation. The Vatican verifies these miracles and also looks at other unusual occurrences, such as the body of the proposed saint being free from decay. Finally, beatification and canonization take place, and the person is officially recognized as a saint.