Stock up and keep your Repellent Appliance full with the Thermacell Refill Packs. Each pack comes int 3 mats and 1 butane cartridge giving you a total of 12 hours or protection and repellency. To continue to repel mosquitoes it is advised that repellent mats should be replaced after 4 hours of use while butane cartridges will last you the full 120 hours. They are simple to install and takes seconds to do so. Pick up a couple extra in case your going to be out for a while and don't let anything "bug" you on your next day out!