What is the maximum length of Portable Recorder to Camera cable that is safe to use?
Without more detail this is not directly answerable. The short answer is, the shorter the better, but the long answer depends on many factors.  


The amount of electromagnetic noise in a room will impact how much the cable picks up noise by working as an antenna.  
The type of cable being used will impact how much shielding prevents cross talk (between conductors if shooting stereo) and acting as an antenna.  Features like balanced signaling will also greatly reduce the amount of noise picked up over the length of the cable.  
The signal type coming out of the recorder also matters.  If the signal is low power (mic level) then it will be more susceptible to noise, but less susceptible to cross talk.  If it is powered (line level) then it will be more susceptible to cross talk but will be much further above the noise floor. 


Conversely, for just about any case, the impact of cable length is unlikely to be as significant as the quality loss expected for sound dissipating through the air, so getting as close to the subject as possible is pretty much always ideal.  

For a long cable run, the wired solution will always be lower latency than a shorter one.  Latency is introduced primarily by processing or by signal dropouts on a digital signal that supports retransmission.  Analog signal on a cable is going to move at the speed of light while the sound waves themselves will have to travel at the speed of sound, so if your camera is in the back of a large room, the sound from the mic coming over a wire will likely actually get to you before the physical sound waves reach you.

There may be some exceptions to the noise level rule if the room is extremely EM noisy and you don't have the option of using balanced and shielded cabling with a line level signal, however sound drops off in strength EXTREMELY quickly as the pressure waves spread in all directions and lose power at an inverse square level. (Inverse square law).  The closer you are, the more signal you have to start with and it's difficult for a cable to lose signal that quickly.