it depends on what you are feeding.
if you feed something that totaly digest within 12 houres there is no way u should fast on top of that
with kibbles and grind meat you should never
in the winter feed twice a day instead of thinking about fasting
most people who give their dogs a fasting day, for example they normaly feed 600 gram, they feed them like 1000/1200 gr and then skip a day.
jmo