The 90-day window refers to the period of new vehicle ownership during which protection can be applied to a surface that has not yet accumulated the damage, contamination, and environmental exposure that makes later protection less effective.
The chemistry behind it: in the first 90 days, the factory clear coat is at its maximum thickness. No correction has been performed, no accumulated contamination has etched or embedded, and the paint surface is in its closest-to-original state. Protection applied to this surface bonds to factory-fresh clear coat. Protection applied at 24 months bonds to clear coat that has already been weathered, contaminated, and potentially thinned.
The 90-day framing is a useful heuristic, not a hard cutoff. Jesse has protected vehicles far outside that window with excellent results. But the sooner protection is applied, the more of the original surface condition it preserves.