I don't think there is anything wrong with keeping the block heater plugged in all night but they only need an hour or so to bring the engine up to temp.

I have not needed to use a block heater for quite awhile (garages are wonderful) but when I did I would plug it in when I got up and after my shave, shower, breakfast and coffee the engine block was warm to the touch. I don't think they need to run for hours and hours....

Tim