How Long Cables From Turntable to Preamp?

1.2 meters is the standard tonearm cable length.

As Dan points out, longer can diminish already small cartridge output, and simultaneously increash RFI and hum pickip. So it's the worst of both worlds.

In addition to Dan's very good suggestion of locating the phono preamp near the TT and then not having to worry about the distance to the main preamp, you might want to consider mounting a TT wall shelf (like a Target) right over or next to your rack. IMO, unless you have a very solid rack and a concrete floor, this is the best way to install a TT anyway, and then you could have it close to your other equipment without adding another rack.

I'll grant this one exception, which is: if you're using a high output moving coil cartridge (that means 2+ mV as opposed to 0.2 -- 0.6 mV), you could probably go six to eight feet without much problem.

MM cartridges, although they also have higher output, are susceptible to cable capacitance build-up over much more than 4 or 5 feet, so even though they have sufficient output voltage to drive a longer cable without noise, you will start to get non-linear frequency response problems unless you add capacitors to compensate.