Muscle glycogen is a store of glucose in your horse's muscles and this glucose is the only fuel muscles can burn during high intensity anaerobic exercise.
Horses are useless at replenishing their muscle glycogen compared to humans taking 2 to 3 times longer than it takes us.
In knowing this, the most important factor in determining how well a horse will replenish its glycogen seems to be hydration. A well hydrated horse will replenish its glycogen faster than a dehydrated horse.
Feeding 'grain' probably helps in the 48 to 72 hour time range. Unfortunately the main study conducted only looked at cracked corn.