72 hours
I know you're not personally making that claim -- the company who puts these out do -- but they only stayed cool for us for four hours, max. We used them in that September trip I spoke of earlier in this thread when it was very hot.
Also, the seams don't last long. By the second day of wearing them, expect to have little plastic bits and pieces (the stuff from the inside of the tie) on your neck.
Lastly, I, too, bought them for everyone in my family. Don't expect the guys to wear them. There was no way my husband and brother were going to wear a cute little neckband tie to a place where family pictures were going to be taken endlessly of them.
I did end up using the two that DH and my brother wouldn't wear on a subsequent trip since the ones I bought for myself and the other women in my family started falling apart so soon and had to be thrown away. They do keep you cool for a little while.