Maximal Cable length for a given Opamp
For relatively short cables, capacitance just adds up with length.
If you deal with relatively long cables (longer than wavelength/10), you have to properly terminate the end (I.e. a 75ohms cable needs a 75 ohms termination impedance).
If you don‘t do that, then you will get different impedances at different cable lengths. The cable could e.g. be purely inductive at 3m and purely capacitive at 2m.
You would need to measure the impedance of a cable of a given length.
So there‘s no max. length of the cable in your context. Just some lengths that may have (too) high capacitive loading because you have a mismatch at the cable end.
BTW: The impedance of the cable does not change (much) over frequency if properly terminated. Just over length or frequency if not properly terminated.