I read somewhere that more or less each month a new data dump is available. The last dump for Stack Overflow was on February 9, 2011. So when can I expect the new data dump?
The data dumps now only come out every two months, so you can expect a new one just after April Fool's Day. SEDE is still updated monthly, though.
Because the dumps are quite a bit of work for us, we’re moving to a bi-monthly schedule instead of monthly. Meaning, you can expect dumps every two months instead of every month. If you have an urgent need for more timely data than this, contact us directly, or use the Stack Exchange Data Explorer, which will continue to be updated monthly.
As of September 2011, it's every three months.