Answer: 1.8 zettabytes - or 1.8 billion terabytes.
The massive numbers come courtesy of IDC and EMC's fifth annual Digital Universe Study. In an attempt to put that number in some perspective, the press release states that 1.8 zettabytes of data is equivalent to:
Every person in the United States tweeting 3 tweets per minute for 26,976 years nonstop. Or, in iPad terms, it is enough to fill 57.5 billion 32 gigabyte iPads, which would be enough iPads to build a 20-foot high wall around South America.