The decision itself can happen on the day of travel at a port of entry, but the preparation rarely takes less than several weeks. Redrafting duties, obtaining a transcript or licence certificate and resolving any ownership question all take time.
The delay sits in the paperwork, not in the decision
Preparation is the slow part and it is the part within Marc-Olivier's control. A university takes time to issue an official transcript, a regulator takes time to confirm registration in good standing, and an employer that has to rewrite a job description will not do it overnight. An adviser then needs the finished package early enough to say that a duty description reads as supervision rather than forestry, which is a fixable problem weeks out and an unfixable one at a booth.
Where the employer prefers to file Form I-129, add USCIS processing, and remember that travelling before the approval arrives wastes the point of petitioning. Field seasons create their own pressure: an employer that needs someone in the woods for a specific window will resist a slipping start date, so agree the schedule inside the offer rather than around it.