Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Five hours? Maybe five minutes.

I bought three pairs of these in preparation for an event that would require various family members to stand outdoors in sub-freezing temperatures for several hours. Two of the pairs warmed slightly, not really a measurable feeling of heat -- more like lukewarm -- for less than ten minutes. The third pair was actually toasty for about a half hour, and then the heat dissipated.There's no way of knowing whether this item will work or not until you use it. We were counting on the warmers to keep our feet from freezing, and instead suffered a painful disappointment.