This site uses this specific heading for its guarantee section: "Two Months to Decide" — worth understanding precisely what that framing describes.
This site directly connects the 2-4 week timeline to notice improvements with the full 60-day guarantee window, describing the guarantee as giving "real room to judge whether it's working for you."
Since most users are described as noticing improvements within 2-4 weeks, the full 60-day guarantee gives genuine room to judge results within that stated window, rather than deciding on day one or two.