An official explanation on this issue can be obtained at the Turkish Embassy .Their answer is 3 months .But on the memory of none of the operators there is no case that tourists were deported from the country on arrival .This is due to the fact that the border guards at passport control (at least in Antalya) have an indication to let tourists with the validity of the passport coinciding with the end of the trip (you may need to present a voucher of .But "Aeroflot" or "Turkish Airlines" ticket offices do not have the right to sell a / b for citizens whose validity period is less than 3 months .