El Niño and La Niña episodes typically last 9 to 12 months, but some prolonged events may last for years. They often begin to form between June and August, reach peak strength between December and April, and then decay between May and July of the following year. While their periodicity can be quite irregular, El Niño and La Niña events occur about every three to five years. Typically, El Niño occurs more frequently than La Niña.