brucelanthier,

I think the butane cartridge is rated to last for 12 hours and the mat (with the active ingredient) for 4 hours. That's why the kits come with 1 cartridge to 3 mats.

They get about the advertised hours. The effectiveness does start to decrease as it gets closer to 4 hours. You can usually tell when the mat is used up b/c it turns from blue to white.

Depending on how bad the bloodsuckers are, I sometimes only use it near dawn and dusk, and then I only use 1 mat for a whole day. You can reuse the mat if you use it the next day or so but after that it dries out.