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If the tenants never turned in their keys, then they would still be considered to be in legal possession of the property until you took action to end any access that they might have had. So here, that would be when you changed the locks so that any keys that they kept would then be inoperable. Turning in the keys is the primary indicator that they are surrendering legal possession back to the landlord.
So the time when the 45 days would start to run would be the day you changed the locks.