I'm about to record still videos, mostly of people sitting and talking to the camera. And I would like to use internal microphones of a portable recorder with a camera and I do not want to place the recorder on top of the camera using a shoe adapter cause I'm afraid it will record a noise coming from the camera and an operator. I also don't want to use XLR microphone, cause it means additional expenses for me and additional gear to carry. So my idea is to place the record in front of the speaking person. Since there is a distance between the person and the camera, I would need quite long audio cable that will connect the recorder with the camera. So my question is - are there any issues with using long cable, like latency, quality drop or something I'm not aware of and what is the optimal cable length I'm safe to use in this case?
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.
You may want to consider looking into another alternative as well: double-system sound. The (old school) idea being that since you have a separate recorder, you don't need a connection between the camera and recorder. Instead you can record both separately and then sync them in post-production. If you do this, it's highly recommended to use a clapper board or even just clap your hands in view of the camera and near the microphone to make the syncing process easier.
There is also software, like Pluraleyes and some NLE software, that can make the process easier even without clapping, by matching and aligning the waveform of the audio on the camera with the one from the recorder.
This option may not be applicable to you, but since you didn't describe your circumstances (distance, environment, etc), I thought I would mention it as a possibility.
Finally, one other thing to consider when using a long cable that I don't think anyone else has mentioned so far is physical safety. In other words, preventing people from tripping on the wire and falling down or knocking equipment (like your camera) down.