The body of anyone infected by virus X will, after a week, produce antibodies to fight the virus; the antibodies will increase in number for the next year or so  There is now a test that reliably indicates how many antibodies are present in a person' s body
If positive, this test can be used during the first year of infection to estimate to within a month how long that person has had the virus
If a person remains infected by virus X indefinitely, there is no limit to the number of antibodies that can be present in the person's body.