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.
2 weeks
4 weeks
3 months
6 months