What can i do if my dentist told me i had a cavity a while ago. should i be worried?

Sure: You should be worrying that it didn't progress to the point where the tooth now needs more work than it did a while ago. A simple filling may have turned into a root canal.. Can't tell over the internet. Go and have it restored, even if you don't have pain.

Would you worry if: Your car was leaking oil? You had a leak in the roof of your house? You had an unknown growth on your skin? You were coughing for 2 weeks? You felt dizzy? All of these things are best handled sooner rather than later. Merely waiting and doing nothing just increases the chances of bigger problems. You will have no one but yourself to blame when this happens. Get it fixed, please!

Depends!: Depends on the location, the depth, and your homecare. In adults it can take 6-30 months for a cavity to go from enamel to dentin, once it reaches dentin it progresss much faster to the nerve of your tooth. Like i said before a lot depends on your genetics, homecare, and the depth at which the cavity started. Get it fixed and we don't have to guess!