How long before obturation should the calcium hydroxide paste be allowed to remain in canal, if the apexification of open apex is intended?

As a general rule, the treatment paste is allowed to remain for 6 months. The root canal is then reopened to determine whether the tooth is ready for a conventional gutta-percha filling, as determined by the presence of a “positive stop” when the apical area is probed with a file. Often, there is also radiographic evidence of apical closure.  Frank has described four successful results of apexification treatment: Continued closure of the canal and apex to a normal appearance. A dome-shaped apical closure with the canal retaining a blunderbuss appearance. No apparent radiographic change, but a positive stop in the apical area. A positive stop and radiographic evidence of a barrier coronal to the anatomic apex of the tooth.