Do tenants have a grace period to pay reletting fees? If so how long?

YOU might want to specify where you reside... this is an international site... and you used a term that's not commonly used outside of the UK. Laws vary significantly depending on location.

Any question on law needs to include your general locale as laws vary by area. Some quick research defines reletting fees as fees charged by a landlord for letting a tenant out of a lease early (there were other definitions but they don't apply to a tenant).

If that's what you were referring to, then I doubt if there is a grace period, either statutory or otherwise. If that isn't what you were referring to you need to be more descriptive.