A CNN piece from Sept. 2013 quotes an expert at the Pew Research Center as follows:
These figures don't capture variations from one geographical area to another, of course, nor is there a specific projection here for bilingualism. It's interesting to learn that the percentage of Hispanics in the U.S. who speak Spanish will drop to roughly two-thirds in 2020, even as the Hispanic population grows, but knowing this certainly does not matter when an English-speaker and a Spanish-speaker have to communicate and can't. Luckily my experience today did not involve anything serious. I don't like to think about what would happen if the inability to cross the language barrier implicated a matter of life and death."On the one hand, [in the U.S.] the number of Spanish speakers is projected to grow to about 40 million by 2020 (from 37 million in 2011). This reflects Hispanic population growth and a large number of non-Hispanics who will also speak Spanish," said Mark Hugo Lopez, director of Hispanic Research at the Pew Research Center. "But, even though the [total] number of Spanish speakers is projected to grow, among Hispanics, the share that speak Spanish is projected to fall from about 75% now to 66% in 2020," Lopez said.
Note: edited slightly after initial posting.