I would judge by how much the horse needs. But remember because of the sugar content in hayledge, some horses on box rest can't handle it because they aren't burning the energy, my mare went nuts on it. Personally I would feed the horse hay. And as much of it as they wanted.
oh dear,why is she on box rest???
If you buy the high fibre haylage you should be fine
there are different grades!!
give her small nets throughout the day,if you can,and a large net at night,she should be fine as she should not be getting hard feed,just keep an eye on her weight and adjust accordingly