When should you consult a doctor for a second opinion?

When in question. Anytime you are going to have an extensive, or experimental surgery or if you feel that your current doctor is not helping allieviate you of your pain or conditon, i would seek a second opinion. If your doctor is insulted by this, time to transfer care to another physician.
If in doubt. Get a second opinion. Or if it is a major medical procedure with potential risk, then get a second opinion.