How long would it take for a tooth cavity to spread to the heart or brain?

Why are you waiting?: An acute infection from a tooth into the parapharyngeal airspaces or into the cavernous sinus is a direct pathway to the brain, and airways. If this occurs, its life threatening, and you won't have a lot of time to ask questions. If you have a cavity, get it filled or get the tooth pulled. There is no way of predicting which one will blow up and which one wont.

Cavity vs. infection: The cavity, which is a result of decaying tooth structure, is limited to the tooth itself. However, an untreated cavity can result in tooth infection which does have the ability to spread, especially within adjacent head and neck anatomy. This can have serious consequences and has resulted in death in most severe cases. How long is difficult to answer. Have a dental evaluation soon.

Seldom: A cavity in a tooth, although requiring repair, in and by itself seldom results in extension of infection to elsewhere. It is correct, however, that if left untreated, these may extend into the dental root, adjacent areas of the gingival tissues, and elsewhere, but primary extension to the blood or elsewhere from an uncomplicated cavity seldom happens.