Before the offer is accepted. State surveying licensure and any required examination usually take longer than the immigration step, and a start date built around a field season leaves no slack. Ask the employer what happens if the licence is not in hand on the planned first day.
Work backwards from the field season, not forwards from today
Survey programmes are scheduled around ground conditions and construction windows, so the employer's date is often immovable while the applicant's preparations are not. Ask two questions early. First, what does the state actually require before this person can perform and certify the work: full licensure, a comity or endorsement process, or supervision by a licensed surveyor already employed there.
Second, if that process is not complete by the mobilisation date, will the employer hold the position, adjust the duties or withdraw the offer. The immigration step is usually the shorter one; an admission in TN status can be sought once the letter and credentials are ready, and admission is granted for a period set by the officer, with extensions possible while the employment continues. It is the professional licensing and, occasionally, an employer petition that add months.
Build the calendar backwards from the first day on site and put the irreversible steps, resignation above all, as late in it as possible.