My vet told me they need higher protein (%)...corn runs about 10% vs barley's 13% or cottonseed's 24%. I like a mix of the three, about 45% corn, 45% barley, 10% cottonseed (shouldn't give too much cottonseed). Cottonseed and barley for the protein, corn puts on fat. Usual grain increase should be around 2lbs per week, not much more.
How long depends on the cow -and her current body condition. Some cows put everything they eat into their milk, but with proper nutrition CAN start cycling. Got to have a LOT of feed though. Figure on at least 30 days, probably more like 60 before she starts.