It is estimated that 30 to 40% of all domestic cats have been exposed to the feline enteric coronavirus, a common disease of domestic cats. In catteries that figure rises to 80 to 90%. Three quarters of cats exposed to the virus experience no apparent infection. Of the cats who do, they have a mild respiratory infection and become carriers of the virus without symptoms.
Fewer than 1% of all exposed cats develop what is called a secondary fatal disease known as FIP (feline infectious peritonitis).
Once a cat develops signs of the secondary disease which is present in either the “wet or dry form” he will die. With the wet form of the disease, cats often die within 2 months. Cats with the dry form may live up to a year with a decent quality of life. You can read more about the disease if you click on the link below.
In answer, therefore, to the question in the title, the maximum life expectancy is about 1 year but it may be a shorter at 2 months.
Medications can make an infected cat’s life more comfortable. Life may be prolonged with chemotherapy or immunosuppressive doses of cortisone. Vitamin supplementation i.e. vitamin C can be helpful. Some cats benefit from low doses of aspirin which reduces inflammation. A drug called Trental has been used to treat damage to blood vessels. The virus damages capillary blood vessels especially those of the abdomen, eyes, brain internal organs and lymph nodes which results in fluid loss into tissues and body spaces.
Hope this helps.
Source: Cat Owner’s Home Veterinary Handbook. 3rd ed pages 86-87. Buy this book.