There is a short and rude answer to this question. Zero.

The conductivity is the product of mobility and the concentration of charge carrier at the Fermi level. Mobility is known 15000 cm2⋅V^−1⋅s^−1 (from Wikipedia). There are no charge carriers at the Fermi level in a separate sheet of graphene. It has to be doped to shift the Fermi level. Everything depends on doping.

@ Igor yes sir, if chemical potential is 1 eV, and thickness is 1nm then and room temperature what would be the complex conductivity of gaphene in THz range.

Here one paper about it