How long is it supposed to it take for a cavity to start hurting?

It's variable.: Cavities begin to hurt when the decay process penetrates the insensitive enamel and enters the sensitive tooth structure underneath it (this structure is called dentin). The process by which the decay penetrates the enamel varies with: diet, hygiene (brushing, flossing, etc.) genetics (the density of the enamel and dentin), use of anti-cavity substances (fluoride), etc. It will vary.

Too long: Unfortunately, by the time a tooth starts to hurt from dental decay, it's substantially damaged. Everyone has different pain thresholds. Some patients can have the entire tooth destroyed and rotted away and report they never felt anything wrong. Others can sense things after moderate tooth destruction. Brush, floss, eat healthy foods and see your dentist every 6 months for checkups!