After admission, not before. A port-of-entry decision is immediate but can be a refusal; a petition approval arrives before travel. Initial admission is for up to three years and renewals are available, subject to the stay remaining temporary.
Resign last; renew on the facts
The sequence that protects an employee leaving a secure position is to finalize the letter, gather the documents, obtain the admission or the approval notice, and only then give notice. Utilities often require several weeks' notice for senior staff, so plan the port-of-entry date or the petition filing around that rather than the reverse. If the firm files Form I-129, the approval notice fixes the start date and she can resign with confidence; if she applies at the border, keep the resignation letter unsigned until she has been admitted and has the record of admission in hand.
The initial period can be up to three years, matched to the letter. Renewals are made by petition or by re-applying at the border with a fresh letter, and each requires that the engagement remain temporary in character. A pending immigrant process does not automatically bar renewal, but it must be handled openly, which is why the sponsorship conversation belongs in a consultation rather than at inspection.