An analysis from the Pew Hispanic Trends Project shows that (as of 2013) a slight plurality of Latinos living in the US spoke Spanish as their main language, and a slightly smaller number were fully bilingual (using both Spanish and English as main languages). But this varies hugely depending on how long someone's family has been in the US:
A majority of Spanish-speaking immigrants to the US (and from Puerto Rico, which Pew counts as "foreign" when it comes to language use) tend to speak Spanish most of the time. This doesn't mean they don't know English; as of the late 20th century, Latino/a immigrants were more likely to speak English than their European predecessors of a century ago. But Spanish is the language 60 percent of them tend to use.
Second-generation Latinos — the children of immigrants — tend to be fully bilingual; this might mean they're used to speaking with their parents in Spanish but using English outside the home, or just that they're in situations where they deal with Spanish- and English-speakers pretty much equally. And with the third generation, who are grandchildren of immigrants, bilingualism fades quickly. In fact, the proportion of immigrants who speak mostly English (35 percent) is bigger than the share of Latinos who are thoroughly bilingual in the third generation.