I've heard numbers thrown around for anywhere between 6 and 18 months for common antibody lifetime. Obviously, these are guesses based on similar viruses and have evolved over the past year. It's probably going to be endemic, although it's possible that with sufficient vaccine production we can knock it out of humanity. The problem is it seems to survive so well in animal hosts.
Reinfections right now are rare enough that we don't know much, but there have been second cases that seem more severe than the first. This is likely because of lung damage from the first case. But faded immunity from the vaccine won't have the same lung damage. So the faded immunity should be sufficient to eliminate the worst outcomes most times.
So the question of when we start boosters depends on how bad the condition will have to be that we are inoculating against.
