The application step itself can be same-day at a port of entry, but the preparation behind it is what needs lead time. Allow weeks for a registration certificate, a properly drafted employer letter and any restructuring of a self-employment arrangement.
The fast step at the end of a slow sequence
It is easy to misjudge this timeline because the visible step is short. An officer at a port of entry can decide a request in an afternoon. What sits behind it is slower. A professional regulator may take weeks to issue a letter of good standing or to restore a lapsed registration. An employer's human-resources department often needs two or three revisions before a letter describes duties in the terms the classification uses rather than in internal job-family language. If the applicant has been operating through their own company, winding that arrangement down, or changing it so that the U.S. work is plainly employment, takes accounting and sometimes contractual steps. A petition filed with USCIS adds processing time that should be checked at the time, not assumed. Working backwards from a start date, six to eight weeks of preparation is a realistic floor.
Hypothetical example: Elise, a Canadian geochemist, is asked to begin a Nevada mining-project assignment on 1 July. She should first obtain the employer's draft duties and compare them with her academic record, then request any required degree evaluation or licensing material before booking travel. That order matters because a border interview can be fast only after the evidence exists. The officer will assess a temporary professional role for a U.S. employer; it cannot be an arrangement in which Elise is effectively selling her own services through a business she controls. An I-129 filing may suit an employer that needs an approval record before travel, but it adds a separate processing stage. Start dates often fail because a manager announces one before the letter, credential, and employment structure have been checked together.