Fees are due with the online applications; the gifts are paid only when the government instructs, after successful vetting. Between those points sit document submission with Form I-140G, the vetting itself, an EB-1 or EB-2 determination subject to visa availability, and possibly an interview. Official material describes weeks, but no date is guaranteed.
Do not tie the school year to it
Families usually plan around a September start, which is exactly the commitment worth keeping flexible. Avoid paying non-refundable school deposits, ending a lease, or resigning on the assumption that the process will finish by a chosen month. Keep funds liquid enough to respond quickly to a gift instruction, and set review points after filing and after vetting so the household re-checks its plan against what has actually happened.
Add one practical arrangement to those review points. Decide in advance who in the household is responsible for correspondence, where documents are kept, and what the family does if a request arrives while someone is travelling — because a process involving four people generates four times the paperwork and exactly the same capacity for something to be missed. Keep every passport valid well beyond the expected period, and avoid scheduling long absences around points where an appointment or a document request is plausible.
Hypothetical example: a family nominates one person to monitor everything, maintains a shared index of what has already been obtained, and reviews it monthly, so a request for a certified translation is answered in days rather than triggering a search through four sets of folders.