How long it takes to get through a bag depends entirely on how much you give the horse. If it is kept in a closed, rodent-proof container, it should last until the best before date on the bag, or until ithas all been eaten, whichever comes first.
Hard food doesn't go off if kept dry and not allowed to get too hot. Vitamin levels degrade over time but not the calorific value. British oats bought in July, (alone or included in a mix) for example, will usually be a year old already.
If there is added oil or oil based ingredients (like linseed) it will go rancid over time - more quickly if it is allowed to get warm. Check the sell by date on the bag and you should have plenty of time to feed it even at minimal amounts.