Any time your body fights off a pathogen, antibody production ceases shortly after the infection has been killed and about 90% of the B-cells that were producing the antibodies die since they are no longer needed. The other 10% remain dormant as Memory B-cells, which are ready to be reactivated to produce the specific antibody required should that pathogen ever be encountered again.
So to answer your question about what happens after ~6 months? Your body continues doing what it does with every pathogen it’s ever encountered: remains ready to kill it. Should somebody cough a bunch of SARS-CoV-2 at you a year later, your body would respond by awakening those Memory B-cells and the SARS-CoV-2 viruses would immediately be obliterated by your natural immune response. It’s highly unlikely you’d ever experience any symptoms unless you were immunocompromised.
Could I already have memory B cells without taking the vaccine?