You can feed it up to 24hrs after as far as I’m aware.go for the biggest you can afford though as I have the medium Haygain and wish I’d got the bale size
Freshly steamed hay is yummiest, but it doesn't start degrading til 24 hours afterwards.
I've got the Haygain one for my two neds, I was concerned that it may not be big enough but it does great. It's very easy to pop another load on if you need more hay, the cycle takes one hour. It takes a good third of a bale at a time. My neds mostly live out though, so if you have stabled horses then a bigger model may be best.