An article was published yesterday in Nature, SARS-CoV-2 mRNA vaccines induce persistent human germinal Centre response. The time frame is 12 weeks after the booster.

Who current recommendation would be a booster every 2 years for people, 1 year for those with weaker immune system.

I think that this really depends on the evolutionary dynamics of the virus, we have a kind of source - sink dynamics which favour the production of lots of variants (e.g India) and strong selection (countries with people vaccinated).