To be honest, there's no real "cold" for cats like humans get.
Humans, unlike cats, develop a cold from the Rhinovirus. It's short-lived and is self-limiting within a week of symptom development.
Cats can have a few things:
1) The herpes virus, commonly refered to as FHV.
2) A bacterial infection, many sources and many different kinds
Herpes, which we've suspected before in Abby, is long-lasting and does not go away--however, it can lay dormant. Lysine will help out in this department. Most commonly affected are the eyes and upper respiratory tract. Symptoms can last anywhere from 3 days to 30, depending on the severity. Some cases of outbreak can cycle, where it appears there is no improvement at all. Secondary bacterial infection, while immune compromised, is very possible. Especially if the pet is exposed to other pets.
A bacterial infection is spread by inhaling a bacteria that sticks in the lungs. This can be obtained anywhere, including from the owners clothing. Symptoms, for most bacterias, last about 14 days. Antibiotics help in these cases to kill off common strains of bacteria. Some cases are much more developed and, if symptoms last for months, may require a trans-tracheal wash and a culture and sensitivity to determine the type of bacteria and acceptable treatment for infection. Most pets fair well with a bacterial infection, however pneumonia can develop from this quite quickly if improvements are seen. Bacteria commonly sits deep within the lungs generally causing a lower urinary tract infection that may be combined with an upper respiratory tract infection. The severity often determines the length of symptoms.
Note: A common bacteria for cats to have is Bordetella bronchiseptica, often known as kennel cough, which can be obtained from sharing respiratory aerosols with other pets (dogs included). Pets can remain asymptomatic for years before an outbreak. Commonly, this bacteria is picked up while the pet is being boarded, groomed or in a pet store. Antibiotics generally work very well against kennel cough.