last time I checked, the answer was "we don't know". There seems to be a >=8 months figure floating around now, though.

https://www.cidrap.umn.edu/news-perspective/2020/12/two-stud...

We have only had the vaccine for 9 months so there's not enough data to know.

With common cold coronaviruses we see that reinfection is possible for many people after a year, though they don't seem to develop symptoms if they get it again so quickly. I'm sure if you look at enough people you'll see a wide variation in response though.

Natural anti-bodies to other coronaviruses in the same group seem to last more than a decade. People who had SARS in 2003 still show an immune response 17 years later. There's a good chance this vaccine is a one-time thing or perhaps just a couple of times in your life.

https://gulfnews.com/uae/how-long-does-immunity-to-sars-coro...

https://www.nature.com/articles/s41586-020-2550-z_reference....