how long does a section 21 last before it expires

A section 21 was issued to a tenant who was on a periodic but behind in rent. She has since paid arreas off in full so I agreed she could stay in property and remain on a periodic.

If for instance, sometime in the future the tenant again started to not pay rent, would the existing 21 notice be valid or would I have to issue another 21 notice.

Thanks in advance