Ours have the metal plug in ones in the garage and seem to like them = they are on all winter from about November onwards - our single garage is where some of our outside ferals sleep - it contains 4 armchairs and they have beds in the rafters - it also contains my wood shredder and some dried logs for human heat
In the wnedy house at the bottom of the garden I use the micro heat pads - 3 off and the ones down there seem to like them to - we get about 11 hours of heat from them - I am the idiot at 11pm in the evening with 3 foot of snow wading down to opo them in - whilst walking the dogs!