If it is merely seasonal allergies you should be fine. If you are under allergy immunotherapy (allergy shots) then you have to be completely finished with them one year before you would report as a cadet.
Remember, the issue with many waivers is not just an issue of USMA. It's an issue of what you will be doing when you graduate, of being able to handle an austere environment where you have no idea what is being used to cook the food. Ask the third world cook at a FOB DFAC in Iraq or Afghanistan if the food is allergy safe and you'll just get a blank stare.