Several words to describe mine:

Random --- Happens anytime for no reason
Frequent/Periodic
Situational --- If I encounter a sort of life event, such as discovering an important answer to a serious life issue, passing of loved ones, heartbreaks, supports, etc....these important little things are the factors that triggers everything in me.

There's no word to describe how long my manic/depression would last, boils down to the 3 factor above.