It's fairly easy to get a rough estimate from publicly available data.
For example, http://en.wikipedia.org/wiki/Decay_heat
says "After one year, typical spent nuclear fuel generates about 10 kW of decay heat per tonne, decreasing to about 1 kW/t after ten years."
So, let's consider ten year old spent fuel. One 1 kW/t is 1 W/kg = 1 J/(s*kg). Since all this heat is produced solely by radioactive decay, it means that radiation inside spent fuel pellets is 1 Gray/s. (In other words, it's still insanely radioactive).
Since after 10 years most short-lived isotopes have decayed, most of this radioactivity is caused by 30-year half-life isotopes Cs-137 and Sr-90 (see http://en.wikipedia.org/wiki/Fission_product_yield
). Sr-90 is pure beta emitter, so if fuel is intact, it goes not give off much gammas. Gammas are mostly from Cs-137.
Very roughly, we can assume that about half of the radiation is produced by gamma-active Cs-137. Decay energy of Cs-137 is 1.176 MeV. 1 MeV is 1.602*10^-13 J. Thus decay energy of one decay is 1.884*10^-13 J. In order to generate 0.5 J/s (0.5 W), how many decays of Cs-137 do we need? About 2.5*10^12.
That's the answer you seek: the gamma activity of intact spent fuel after 10 years cool down is about 2.5 Terabecquerels per kg.