Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Council in Trullo; Often Called The Quinisext Council.: Canon XXV

Canon XXV.

Moreover we renew the canon which orders that country (ἀγροικικὰς) parishes and those which are in the provinces (ἐγχωρίους) shall remain subject to the bishops who had possession of them; especially if for thirty years they had administered them without opposition.  But if within thirty years there had been or should be any controversy on the point, it is lawful for those who think themselves injured to refer the matter to the provincial synod.

Notes.

Ancient Epitome of Canon XXV.

Rural and out of town parishes held for thirty years may be retained.  But within that time there may be a controversy.

Compare notes on canon XVII. of Chalcedon.