The longest a car can go without being turned on or sit without being driven is around 2 to 6 months before the battery goes dead and may not start back up and the battery may not hold a charge anymore.
Also after around 6 months to a year the tires may start developing flat spots or wear from sitting too long without being driven and moved around and if sitting on soil the cars tires can sink into ruts in the ground.
Technically though a car can sit for decades without being started or driven and can start and operate again.
Although the longer a car sits the more problems the car will develop over time and then it may require cleaning of the fuel tank and fuel system to get the engine to run again.
As gasoline sits in the fuel tank the gas can start to go bad after a month or two so it's best to start it up at least one a month and move it even just slightly to protect the tires.
I've had a car that sat for 10 years before and it surprisingly started by needed some fresh fuel and fuel system cleaner.
If not driving your vehicle for more than a week you should put a battery tender on it to keep the battery slowly trickle charged to keep the battery from going bad.
If you don't drive your car for a year the battery will go dead and may not be any good anymore especially if you don't have a battery tender on it or trickle charger to keep the battery charged.
Also if a car is not driven for a year and sits in one spot the tires can start to degrade and get flat spots and if on dirt the cars tires may even sink into ruts into the ground.
Moisture can also accumulate in the engine and engine components and lead to rust and corrosion and fuel or gasoline that is left in the tank can turn bad and turn into varnish and cause clogged fuel filters, clogged fuel lines, clogged fuel injectors and make it hard or difficult or impossible to start the vehicle.
In severe enough cases if the gasoline goes really bad in the cars gas tank you may have to drop the gas tank and and flush the old gasoline out of the tank and then clean the gasoline tank and put fresh gas in.
A vehicle that sits for longer than a month should have some gasoline in the tank so that you can start the car or vehicle up at least every few weeks.
This keeps the gasoline fresh through the system and also starting the vehicle up allows the battery to charge up and allows fluids like oil and coolant to flow through the system and also keeps the seals livened up.