Thank you for question & welcome to just answer.
I will answer your query one by one!
Your 1st query- how long does it take to get a cavity?
Dental plaque which is triggered by bad oral hygiene and other factors such as diet rich in sugar (complex carbohydrates) can easily trigger the bacteria to act on the tooth surfaces and degrade it.
Read more about it here: Read more about it here:
Read more about dental caries/cavities here:
Coming to your second query- Is it possible to get a cavity under a bridge?
Yes it is possible to get a cavity on the area where the bridge meets the tooth. A dental bridge is difficult to keep clean. If the margins of the bridge are not sealed properly ( i.e are opened ), it allows bacteria and acid to undermine the tooth. Mostly toothbrush bristles will not reach under a bridge which triggers cavity formation and tooth decay.
I hope this answers your question. Please Feel free to contact me again for more information on this topic or if you have any doubts or related queries.
I wish you well!