La Nina typically lasts longer than El Ninos

http://www.smithsonianmag.com/smart-news/say-goodbye-historic-el-nino-get-ready-welcome-la-nina-180959178/?no-ist

When La Niña arrives, the U.S. Southwest, central and southern Rockies, the Great Plains and Florida usually have drier weather. Winters under La Niña are likely to be colder in the Northwest, northern California, northern Intermountain West and north-central states. However, although experts have noticed those trends, they have trouble predicting exactly how any one El Niño or La Niña will play out.   

"[As] still-parched Californians found out this year, each event is different, and not all of them affect the weather as we would expect," writes Mersereau.  

Experts get some idea about how the pattern affects global weather and for how long by looking at past events. Becker writes that there are 14 La Niñas on record:Of those 14 La Niñas, nine immediately followed El Niño years. Two occurred two years after an El Niño, with a neutral year intervening. Two were the second year of a “double dip” La Niña, where sea surface temperatures briefly returned to neutral during the summer before heading back into La Niña territory (1974/75 and 2011/12). The remaining one starts the records off in 1950.La Niñas also tend to last longer than El Niños. After the unusually strong 1997/98 El Niño, La Niña lasted for 33 months, spanning three winters.