The amount of time, usually referred to as "airport" or "garage" time, that you can leave your vehicle parked and still start your engine is dependent on such things as the battery's initial State-of-Charge (SoC), the Reserve Capacity, the amount of natural self-discharge and parasitic (ignition key off) load, and temperature. Car manufacturers normally design for at least 14 days or more "airport" time; based on a fully charged battery in good condition, moderate weather, and no additions to the original car's parasitic load (for example, an after market alarm system). The number of days will vary based on the temperature. When a battery drops below 100% SoC, permanent sulfation starts slowly occurring, and this will reduce the capacity of the battery, and left unchecked, will kill the battery.
If you leave your vehicle parked for more than two weeks, then you have several options:
15.1. The best option is to connect a three or four stage "microprocessor controller" battery charger, voltage regulated float charger, or solar float charger and continuously float charge your car battery. You will need a "float" charging voltage between 13.2 and 13.8 VDC at 80° F (26.7° C) and at least .5 amps (500 milliamps) to overcome the vehicle's parasitic load and the natural self-discharge of the battery.
15.2. Disconnect the NEGATIVE (-) battery cable to remove the parasitic load, but be sure that you have saved any security codes or radio stations presets that will have to be reprogrammed. The battery's natural self discharge will continue.
15.3. Replace the battery with an VRLA AGM or Spiral Wound AGM type battery, e.g., an Optima or Exide Select Orbital with very low self-discharge rates. For periods greater than three months, also disconnect the NEGATIVE (-) battery cable to remove the parasitic load.
15.4. Install a battery with a larger reserve capacity or connect an identical battery in parallel.
15.5. Replace the battery when you are ready to drive the vehicle again, especially if the battery is over three years old and in a hot climate.
15.6. Have someone drive your car during the day at highway speeds every two weeks for at least 15 minutes to keep the battery charged.
15.7. Jump start the battery and hope that there is no latent damage.
15.8. Install a low voltage disconnect. This is especially helpful if the driver forgets to turn the headlights off.