I'd guess it depends on soil and time of year, where you are located (down south still growing, scotland prop not), and when it was last grazed/ how long the grass is. If you want the ground to last all winter I'd be tempted to put hay out sooner than 'needed'. If you put out half a bale, if there is enough grass they will ignore it - so don't put any more out! When the start to eat it, carry on topping it up then they won't eat the grass down to the roots and the field overall will cope better.