How likely is someone to go 2-3 years without a single manic depressive episode with no medicine?

Possible: It is usually more common to have frequent episodes, but it is not impossible. Bipolar individuals on average have 4-7 episodes per 10 years. These episodes can last several months.

Agree w Dr. Howell: Although this may not be the norm - it can happen. Bipolar Disorder can express in a number of ways. There are some people who have "normal" energy level & mood most of the time with sporadic depressive & manic or hypomanic episodes. Others continually cycle back & forth between mania, hypomania and depression.