Tests this year on blood from people who had SARS in 2003, indicated that 17 years later they still had T-cell responses . On the other hand, coronaviruses which are experienced as "colds" apparently have a lot less retention of immune system recognition. Optimistic scenario is that the immune system is "smart" enough to recognize which infections are a big deal which must be remembered forever, and which are not. Pessimistic scenario is we got lucky with SARS, and may not with Covid-19.
Immunity isn't a binary condition which lasts for a certain length of time and then stops. It exists on a spectrum. The best evidence we have indicates that most recovered patients will retain a significant level of immunity for at least several years.