https://www.medrxiv.org/content/medrxiv/early/2021/03/02/2021.03.02.21252203.full.pdf
While we don't have any official word on what antibody levels are needed for protection, we do have some data that would allow us to make an educated guess.
Specifically, we have several studies showing that people who have antibodies from infection have reasonably good protection against another infection, although not as good as the RNA vaccines. And we have one study done in healthy young men (military recruits) showing very low rates of reinfection in those with antibodies, as long as the antibodies were above about the 10th percentile. That is, only the 10% worst antibody levels were not really protective, at least in young healthy people.
If you are in the US, you can get a blood draw and have a semi-quantitative antibody test done. It will pick up antibodies from either vaccination or infection. The catch is that it maxes out at 250 (arbitrary units). In the UK they are using this test but diluting it tenfold, then multiplying the number they get by ten. This is a legitimate way to find higher levels of antibodies, but as far as I know, it's not available in the US.
The graph above shows this test done on samples from people who had covid.
If you do this test and the result comes back >250, you know that you have reasonably good immunity. You might get a breakthrough case, but it's very unlikely to be severe.
The 10% threshold is somewhere between 10 and 50 (it's a log scale, so hard to eyeball). Below 50 or so is the point where I would start to get concerned about not really being well-protected against severe covid, because levels do decline over time. So a person who is at 50 today might fall below 10 within four to six months. A level of 50 would be considered in the low-normal range for someone who had covid two months ago.
Bear in mind that this is just my best guess.
People who get the RNA vaccines typically have antibody levels of about 2500 using this test (which is maxing out the UK version). So if you have a healthy immune system and you are vaccinated, you will max out the US version of the test. It can't tell you where on the spectrum of normal responses you fall because even low-normal responses to the RNA vaccines will come back >250.
The people who will find this test useful after vaccination are those who are immunocompromised in some way and therefore might fall dangerously below the range of normal responses.
The way to get this test is to have your doctor draw blood and order Labcorb 164090. Most places you can also go to Labcorp directly and they will charge you $10 to take blood. The test itself should be billed to your insurance company.
Here is the Labcorp page for this test:
https://www.labcorp.com/tests/164090/sars-cov-2-semi-quantitative-total-antibody-spike