There is no universal waiting period in this overview that makes a filing or travel sequence safe. The actual intent, status history, steps already taken, and proposed actions matter. Ask an immigration attorney to assess the facts before scheduling a filing or a new entry.
Use a dated sequence for review
Provide current I-94 details, assignment dates, planned trips, and any employer or family immigration activity. Explain changes as they occur, because advice based on a tentative future idea may not cover a concrete filing decision. Do not delay seeking advice until an expiration is close.
A TN approval does not promise later renewal or permanent residence, and a future application should not be assumed to preserve current work permission. Because the current admission record governs, anchor the sequence to that record rather than to the calendar. The authorised period, the assignment dates and any planned trip form a chain in which each link constrains the next, and an extension is generally prepared while status is still valid rather than after it lapses.
Hypothetical example: a Biochemist schedules a research trip abroad partway through an assignment and treats the return as automatic; re-entry is in fact a fresh admission request decided on the evidence carried at that moment, so the file has to be current before departure rather than rebuilt afterwards. Fix the order of the remaining steps around that constraint.