Preparing for survival situations never ends as many situations, both man-made and natural can happen at any time, usually without much warning. It's important to make sure you have the essentials your family will need to survive.
Even though some power outages only last between 2 and 24 hours, there have been instances where the power did not come back on for 3 months. In recent months, power has been out for many weeks due to earthquakes, winter storms, and hurricanes. Preparing for months of sustainability is not much harder than preps for 72 hours.
The most important supplies are food and water, but long term planning requires more than just the survival basics; you must be able to thrive. Plan for an abundance of water and a widely varied diet, keeping in mind that financial independence is important.
Long-term water supply. This is a question of infrastructure; a continuous supply is the issue. You’ll find specific details in the Emergency Water Supplyarticle. A friend of mine is digging a well on his property in case the water utility is compromised, and he’s setting it up to work on electricity and as a manual system. Other options include a solar-powered pump and a wind-powered pump. Either option would entail a cistern to get him through the dark, or windless days.
Contingencies: Set yourself up to use public utilities as well as your backup systems. Modern conveniences are very convenient; use them for your benefit, but be prepared to thrive without them.