There is no universal milestone in this overview that makes a move safe for every applicant. Ask counsel what status and documents your intended move requires.
Work backward from actual permission
Use tentative housing or school plans while tracking outstanding agency steps. Revisit the plan when a substantive notice arrives, rather than treating an estimated processing interval as a deadline binding the government. An expedited process can still involve additional evidence, an interview and availability questions.
Work backward from actual permission rather than forward from an application date. The milestone that governs a move is the point at which the applicant holds a status that permits them to be where they intend to be and to do what they intend to do; everything before that is preparation, however encouraging it looks. Identify what document would evidence that permission and treat its issue as the trigger.
Then work backward to establish what must happen beforehand and how long each step realistically takes. Avoid using an application date, a payment, an acknowledgement or an adviser's expectation as the anchor for a resignation, a property decision or a school withdrawal. Where a date must be given to somebody, give a range with the dependency stated rather than a date with none.