When heated, THE FLAX PACs heating pads will retain its heat for approximately 35-45 minutes.  Longer under a blanket or covering.  Our FLAX PACS are filled with flax seed.  Absolutely no fillers whatsoever.  For this reason, the flax seed, a tiny seed, will hold its heat with the denseness of its packing.  We often tell our customers to give their FLAX PAC a shake before they go back to the microwave.  The heat trapped in the density of the seed will resurface and keep your pack warm for another few minutes.  There is another question to be answered in how long microwavable heating pads last.  That would be determined in days, months and years.  My best answer is THE FLAX PAC will last at least 4 years because that's how long we've been in business and we haven't had a customer tell us their FLAX PAC needs to be replaced!  We are very proud of that!