Roughly 0.18 to 3 mSv per annum [0]. (EU regulations limit it to 3 mSv/a maximum [0]).
One banana is 0.1 µSv [1].
The granite is therefore 0.18 mSv / 0.1 µSv = 1800 bananas.
This is compared to 4 mSv normal yearly background dose [1]. The granite increases background radiation by 0.18 / 4 * 100 = 4.5%. The yearly dose from natural potassium in the body is 0.39 mSv, so I wouldn't worry about the extra 0.18 mSv if I were you.
You're at much greater risk if you live in a house which used reinforced concrete with steel from the Ciudad Juárez accident - after that accident, 109 houses were demolished. [2] [3]