Starting as soon as possible is the best way to plan for a long term food shortage. Planning for around 6 months of food supply is a good starting point. Disasters of the past like Hurricane Katrina left families in search of food for a couple years, at least. While food aid from the government could help, they mostly supply foods that are high in sugar and salt content.
Starting as soon as possible and strategically planning for months in advance is best. Creating lists of recipies to cook at home and what you would need for them is a good way to determine how much you'll need as well. Greenhouses and gardens can provide constant supply of veggies and fruits, but freeze drying meats has the potential to last for 10-15 years when done correctly. This means full meals with plenty of protein can be planned for long term shortages.
