We know that 6 months after having COVID-19, most people still have antibodies, so there is no reason to think that it would be different with a vaccine.
As for how much longer will the antibodies still show up, well, we’ll have to wait to see when those who first had COVID-19 finally show no antibodies.
Antibodies aren’t the only armour in the immune system though, T-cells etc., are also on call.
We can do booster vaccinations too if needed.