Yes, this has been suspected for a few months, and confirmed by several studies within the past month, that antibodies remain in the system for 30 to 90 days, after which the patient can be reinfected.
Also, it's still not completely well known at this point how long immunity lasts to this virus. Having neutralizing antibodies drop down to undetectable levels after a while is actually quite normal. Antibodies only form part of the immune system's defense against pathogens. There are CD4 helper T-cells, which are stimulated by memory B-cells, that can reactivate the body's production of antibodies against a specific pathogen. A great example is measles- whenever someone is exposed to measles, either by infection or vaccination, the level of antibodies in the blood drops down to an undetectable level within 6 months to a year. However, that person is known to be immune to measles for the rest of their life.
Interesting side note about measles and antibodies. My mother, who has since retired, was a nurse for many years. When she was a child, she has a case of the measles and made a full recovery. When she went into nursing school, she was required to show proof that she either had measles or was vaccinated against it. She couldn't find her medical records, so they gave her an MMR vaccine. A few years later, she started her first nursing job, and, in the process of moving, lost her vaccination records. So her employer made her get another MMR vaccine. Fast forward many years later, and she started another job, this time at the Mayo Clinic- and, same issue. She lost her vaccination records, and instead of getting another injection, she asked if her antibody titers could be drawn. The technicians explained that, with measles, the antibodies always fade away, but the immune response remains. So, she had another injection.
But here's where things get interesting in her story. When they heard that she supposedly had been exposed to measles so many times, they asked her if they could take a blood sample a few days after her vaccination. When they ran the antibody test, they were absolutely astonished. A normal antibody response to measles produces 12 copies of antibodies per mL of blood. Her concentration was 230 per mL! Basically, her immune system not only remembered measles, but every time she was exposed, her body mounted a stronger defense.
There's a lot that still needs to be learned about how the T and B cells respond to this infection, and how long immunity lasts to this virus. But from a worst-case scenario, common coronaviruses can reinfect people after about 1 year. T cell responses to the original SARS virus have still been detected 17 years after infection. I think this virus will fall somewhere along that continuum. Not only that, even if someone were to be completely reinfected, their immune response would probably be much stronger, thus giving them a much more mild infection. That normally works even with other viruses, such as influenza, where, despite their rapid rate of mutation, there tends to be cross-immunity after exposure to a related virus.
On a final note- the 1889-1890 "Russian flu" pandemic is thought to actually have been caused by a coronavirus as well. The symptoms that doctors described back then were eerily similar to today's SARS-CoV-2, such as severe pneumonia, gastrointestinal symptoms, and, most strikingly, loss of smell. The virus that is thought to have caused that, Hu-CoV-OC43, has been shown to have diverged from a similar coronavirus which infects cattle in Central Asia. The date that it was estimated to have entered the human population? Circa 1890. Today, that virus causes mild bronchitis.