Early enough that the letter, dates, and credential records are ready before the design phase begins, not before the term begins. Working backwards from the earliest date any employment duty is expected is more reliable than working backwards from the first class.
Anchor the timeline to the earliest work date
Set the deadline against the start of course development. Departments often finalize appointment paperwork on an academic calendar that assumes the term start is the relevant date, and that assumption is what creates the gap. Flag the earlier date in writing, ask when the letter can be issued, and avoid committing to a development start that the documentation cannot yet support.
Work backwards from the state licence rather than from the border. A complete portfolio may be ready while the credential definition or experience record remains unresolved. If the college wants USCIS to adjudicate first, the petition should be filed early enough for regular processing, or with premium processing if the calendar is short. TN requires Canadian or Mexican citizenship, an eligible USMCA profession, satisfaction of that profession's credential rule and prearranged employment that is not self-employment.