Trying to figure out how much water you should have on hand as you prepare for a hurricane or other disaster? According to FEMA, you should store at least a gallon a day per person for a 3 day period in disaster preparedness. After having been through more than a few hurricanes, we'd recommend storing a 10 to 14 day supply. If your municipality loses power, or flooding compromises the water system, tap water won't be safe to drink. Sure, water and ice will get trucked in by the government around 3-5 days after a disaster strikes.