A vehicle has to sit on private property for at least 48 hours which is 2 days before it can be considered abandoned on the property.
If someone leaves there vehicle on your property longer than 48 hours without notifying you then you can take ownership of the abandoned vehicle.
However if someone has broken down and left it in the roadway then after 24 hours the vehicle can be towed and seen as abandoned.
A police officer will put a sticker on the vehicles windshield if they see it on the side of the roadway and no one is there.
Then you usually have up to 24 hours to get the vehicle off the roadway before they will have it towed to an impound lot.
The reason they do that is to make sure the roadway stays safe and to prevent someone else from running into the vehicle.