Prior SARS-CoV-2 infection is associated with protection against symptomatic reinfection.
Please note for the poor readers (most people): the study says “at least” six months, meaning that was the resolution of the study period, not that immunity only lasts six months.
My estimate given how similar past viruses work is significant (in the colloquial sense) symptomatic reinfection immunity in ~90% of people for 2-3 years after initial infection.