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.
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.