He's just turned 31.
Given that India are expected to play around 100 tests in the next 8-9 years and that Harby avgs around 4 wickets a test, can he get to 800 if he plays till the age of 39-40?
Kumble took 343 wickets in only 73 tests after turning 31. (spinners tend to get better in their 30s)
Harbhajan Singh's 400 Test wickets: Stats analysis of his achievement | Cricket Features | West Indies v India | ESPN Cricinfo
He's been avging 32 since mar 2008.
144 wickets in 33 tests (4.4 per match) since then.