How much hay should I give my horse?

My horse has been exiled from his share paddock for trying too hard to be a stallion (haha). So, now I need to subsitute his roughage intake as he is in a grass less stable. I work 8-5 four day a week so I normally feed him a hard feed mixture of chaff, pellets, salt, soyabean, cod liver oil and copra. So, just wondering how much hay he would need and would it be fine to give it to him at night with his evening feed?