This site states this directly, as one of its ten points: a genuine 60-day window — enough time to actually judge whether it's working for you, not just a few days.
This site connects the guarantee length directly to its own stated gradual-timeline claim — worth reading the 60-day window as functionally tied to the "give it real weeks, not days" guidance found elsewhere on the site.
Use the full window deliberately rather than deciding within the first week, since this site's own materials are direct that judging too early works against the formula's own stated gradual approach.