Like many homes in Mexico, we have an underground cistern and a roof mounted tinaco for water storage. Our cistern contains about 5000 usable gallons of water and is designed to supply the house when no water is available from the city but electricity is still available to pump it out of the cistern. The tinaco is smaller, about 119 gallons, but it will supply water even when there is no electricity. Pressure from the tinaco however is very low (6 psi) compared to the normal house pressure which is about 70 psi. City water is about 35 psi in our location which is enough to fill the tinaco on the roof.
Check the capacity of your cistern by measuring the width, length and water depth using a tape measure. Multiply these together, then multiply by 0.00433 to convert cubic inches into US gallons. The tinaco should have a nameplate on it listing its capacity in liters. Multiply this by 0.264 to get US gallons.
The figure below shows the connection between city water, cistern and tinaco. Note the check valve on the far right. Cistern and tinaco water are not purified. Filtering takes place after the pressure pump. Tinaco water does not pass through this path so it is not purified.
Estimate or measure the consumption of all water appliances in the house as described in Water Measurements. Then estimate the lower consumption that will occur under survival conditions. The results should look like the table below.
During normal conditions, the cistern is filled automatically so it should contain about 5000 gallons when disaster strikes. This would last about 5000/112 = 45 days if consumption is not curtailed, but if cut back to the basic needs, it should last 5000/44 = 114 days. The tinaco would only last another 119/44 = 2.7 days.
During the emergency, the water level in the cistern should be monitored at least once per week, and usage adjusted accordingly. With further rationing, the water could last over 6 months.
If you do not have a cistern and filter system, you are probably using Sapasma/tinaco water for washing and sanitation and bottled water for drinking. A garrafone holds 20 liters of water or 5.3 gallons. This is enough to support one person for about 10 days. So you would need 18 garrofones to supply drinking water to two people for 3 months. The tinaco contains about the same amount of water as all 18 garrofones, so it would need to be carefully rationed (one gallon per day) to last for this long. Life would be unpleasant, to say the least, but at least survivable. A tarp could be rigged to refill the empty garrofones with rain water.