Six weeks is workable for a border application if the listing fit is clear and documents are in hand, and tight for an I-129 petition. The real constraint is the days needed to correct a support letter, not the adjudication itself.
Where the calendar breaks
Map the calendar backwards from the day the employer wants work to begin. Hypothetical example: a Lévis automation technician is asked to commission a packaging line at a U.S. plant starting in six weeks.
Week one goes to confirming which listing applies and whether the diploma-plus-experience rule is met. Week two goes to the employer's letter and its corrections. Weeks three and four cover assembling originals, employment letters from past employers and a citizenship document, plus the decision on process.
That leaves a margin for one round of employer revisions, which is where most delays occur. If the employer prefers a petition, add the current USCIS processing time and consider whether premium processing is worth its charge. Do not book a lease or ship tools before the admission or approval exists, and tell the employer early which date is realistic rather than agreeing to one that is not.