How long does it take for a cavity to reach the pulp of a tooth?

Depends: Decay advances because bacteria are given a food source that allows rapid reproducing, excretion of demineralizing acids often insulated from neutralizing and antibodies action of saliva. I have seen early demineralization advance to pulp exposure in unchecked plaque formation with a sugar rich diet in a few weeks. Once the cavity forms between the teeth, floss can no longer clean adequately.

Depends: There is no exact answer for your question as it varies a great deal person to person. Conventional widsom though is that as soon as the cavity has begun, get it fixed right away....Waaaaaayyyy before it gets to the pulp of the tooth! early treatment is very comfortable, much more simple, and exponentially less expensive. So if you have a cavity go now...Not later.

It varies: It varies depending on the person. In some people it can take years and others only months. It is a function of the persons habits an enamel

Varies: There is no answer to your question because this is different for each of us. It depends on the chemistry in your mouth; your diet, oral hygiene, saliva, and the nature of your tooth. In some people decay can progress to the pulp in a few months it in some it can take years. But once decay has begun the only way to get rid of it is to remove it and have a filling or other dental treatment