Nobody knows how long the immunity to the COVID-19 vaccines will last. Some of them may last quite a long time.
The problem is, which of the Covid-19 vaccines you had first and whether you need a booster to it. Some people with the Astra vaccine will need a booster to the Pfizer, just to keep them going for the next year. Maybe even Pfizer, people who have had two dose of Pfizer, will need a booster as well. We just don’t know the answer to how long the Covid-19 vaccine will protect people.
The main reason for the boosters are probably going to be the variants of Covid-19. When new versions of the virus come out, that might be a bit vaccine resistant, or work around the antibodies, and you just need more of a boost.