No more than 2m., it does not matters balanced or unbalanced IC cable. The cartridge very low level and delicated signal as more length it must run in a cable as more degradation suffers.You do want to keep the cable short, but not for the reasons stated here.
The problem is capacitance. The cartridge has an inductance and the cable is a capacitance. Together they have a resonant frequency. Normally its in the RF region or ultrasonic. But with 20 feet it could well be in the audio range. The peak caused by this resonance is often over 30db!!
If you really need to run the cable that far, consider a phono preamp with the turntable, and then run a line signal the distance.