The datasheet of the ATMEGA 64 says 65 ... 260 uS conversion time.
If you use a faster clock for the ADC resolution decreases, so if you are happy with 8 bits resolution (instead of 10 bits) you can use a faster ADC clock.
I think it's maximum 15ksamples/second in the datasheet.
The maximum frequency can also be limited by the S/H circuit. It's recomended to use a small impedance signal source so that the capacitor in the S/H circuit can charge rapidly, if you plan to use higher clock rate.