I have a senior horse who has no chewing teeth, and we feed him a complete ration twice a day. It takes about 15# per day, more in winter, to be a full ration. He is 32 years old, not an easy keeper any more since he lost his teeth, and weighs about 900 pounds, if that helps you estimate how much feed you will need. In other words, maybe a 50# bag will last you about 3 days.
We still leave him on pasture or keep hay in front of him. He grazes all day long and chews on these rolls of grass/hay, and then spits them out and starts that all over. I think it's a mental thing for horses that they need to graze and be chewing on something. That's the thing I would worry about with a horse on full ration pelleted feed. I think if they didn't get their 'chewing time' in, they'd probably chew on anything handy.