To be safe, start your car every three days and let it run for 15 minutes if you are not using it regularly.
Why do this?
Your battery, when connected to your vehicle, drains because power gets sucked even when parked. This is magnified if it has an alarm system, on-board computers and memories for seat positions and radio and climate control settings etc.
How long will it take to drain? That depends on the age of the battery, the climate (the hotter it is) and how often you drive.
So start your car every three days and let it run for 15 minutes to help prevent a no start problem because of inactivity.
