An antibody survey is only of relative use when assessing herd immunity, the immune system is far more complex than just antibodies. You are, for instance, very probably immune to measles, though I very much doubt you will have measles antibodies cluttering up your bloodstream. That also applies to just about every childhood illness.
The assessment of how long Covid immunity lasts is therefore an informed guess. Informed by how long other coronavirus immunity lasts- they are a cause of around 20% of common colds. That is at least a year, possibly two to a particular coronavirus. Covid-19 has no particular difference from other coronaviruses, other than currently we haven’t learned to live with it as anything other than a minor inconvenience.
So taking lower limits, the repeat immunisation schedule should be less than 12 months ideally, or 12 months if you want to put a possibly slightly leaky lid on it, though such risks would be low.
That informed guess might be wrong, of course, but we would find out soon enough. Indeed, we may find out before any decision is made, while the virus is still out and about.