How long can it take for a tooth to decay?

Depends : Baby teeth usually decay very quickly. An adult tooth can take longer depending upon where the decay is located. If the cavity is in the chewing grooves of the tooth it could decay quicker than if it is in between the teeth. Remember, the tooth has a greater chance of breaking when it has a cavity.

Decay: Decay is a result of a multitude of intersecting factors. If you think you have a cavity, check with your dentist.

Caries: On a microscopic level & under the right circumstances, it can take merely hours for dental caries (cavities) to begin. That's why it's so important to brush & floss regularly within an hour of eating. On a macroscopic level, I've seen patients develop cavities in as little as three months (dry mouth, foods high in sugars and\or acids and poor oral hygiene. On a regular basis some every 6 months