In my lifetime: Pew study on population trends says I will be a minority by 2050.

From the horse's mouth:

"If current trends continue, the population of the United States will rise to 438 million in 2050, from 296 million in 2005, and 82% of the increase will be due to immigrants arriving from 2005 to 2050 and their U.S.-born descendants, according to new projections developed by the Pew Research Center.

...The non Hispanic white population will increase more slowly than other racial and ethnic groups; whites will become a minority (47%) by 2050."