FWIW, I came across this today in an on-line article:
It is not yet known how long immunity lasts for recovered patients.
According to the Centers for Disease Control and Prevention, “The protection someone gains from having an infection (called natural immunity) varies depending on the disease, and it varies from person to person.”
“Since this virus is new, we don’t know how long natural immunity might last,” a note on the CDC’s website reads. “Some early evidence — based on some people — seems to suggest that natural immunity may not last very long.”