For a Canadian citizen the port-of-entry route can be decided on the day of travel, so the lead time is governed by document preparation, not advance petition processing. Form I-129 timing depends on current USCIS processing times, and premium processing is available for an additional fee.
Let the credential verification set the calendar
Work backwards from the season. Request the regulator’s verification of the licence or the university’s transcript first, because third-party document delivery can control the preparation schedule. Draft the employer letter next and have it reviewed against the listing before it is signed, because a rewritten letter delays a border application by however long the employer takes to re-sign it.
If the port-of-entry route is chosen, plan the crossing on a weekday with a full day in hand so that a request for more evidence does not cost the start date. If the employer insists on Form I-129, check current USCIS processing times and decide whether premium processing is worth its fee for a field-season start. TN can be granted for up to three years, and a renewal can be sought later; do not shorten the requested period simply to look temporary, because the intent test is about eventual return, not about a short first stay.
A first review creates a reverse calendar tied to the employer’s actual need, credential delivery dates, and the date the letter can be signed. Hypothetical example: a Canadian hydrologist is needed for spring runoff modelling, but her university can issue the final transcript only after convocation. The transcript request receipt does not replace the credential.
Delay the crossing plan or use another documented qualification rather than arrive with an incomplete proof of eligibility.