Patients who receive a negative antibody test after a suspected COVID-19 case might want to get a T cell test since that is another way to confirm previous infection.
https://www.fda.gov/news-events/press-announcements/coronavi...
The half-life of antibody levels has been estimated at 73 days. Good, commonly available antibody tests are quite sensitive, and they're accurate besides (99%), and time has not been enough of a factor yet to produce false negatives for this reason. In a year or two it might be.
As far as I'm aware, there's currently no evidence of people having "weak" antibody responses such that tests don't pick it up in a way that they fall below 99% accuracy.
The only time concern is that you need to wait 2-3 weeks until after you recover from COVID to ensure you test positive, as they take time to build up.