It will keep for a good amount of time if you keep if crotched but not if you take it out of your pants even though the hand warmer is going strong after 2 hours. I took it out of pants and temp went below 90. I put it back in and temp went back to 98. It's going to suck to keep it in pants the whole time I got to do all this paperwork and go over all this crap but got to be done I guess. So according to this I shouldn't have to change hand warmers before I go in to do paperwork. I'll take an extra with me just in case.