In more than 10 years that Ive used synthetics that feature a heatpad, Ive only had one heatpad that was a dud. It had quit working after only an hour after activation. Luckily Ive had a backup, which I had previously purchased at my local camping/outfitters store (which sells them year 'round instead of seasonal).
The rest of the heatpads, as well as the above-mentioned backup, kept on working after 6-plus hours.