it last anywhere from 2-3 hours where the pain is out of control, burning and nothing can touch it. After that, the redness stays for about 4-5 hours. It prickles like a sunburn, that is how I know it is starting. That and I get REALLY hot. I get so hot that I have to pull off as many clothes as I can. I am then HOT for the rest of the day. I went to a wedding on Saturday when it happened, and it was about 10c and everyone else was wearing jackets, and as the sun went down people were wearing parkas and I was standing around in a short sleeved dress sweating like a pig. Everyone thought I was nuts. I hope my Dr. can figure out what this is and get rid of it. I hate it!