If a person started having full blown cold, congestion, cough symptoms, how long would it be best to avoid them for?

You needn You need : not avoid them completely unless you are immune-compromised (HIV, Cancer chemotherapy, etc.), but simply keep your distance. Respiratory bacteria and viruses are typically spread through airborne particles such as a sneeze or a cough which is riskier the closer you are to the individual. It's similar to shotgun pellets: the closer you are the more damage is done. The further away the better.