A car can be left on a jack for several days.
However if you do leave the car on a jack longer than a day then the jack may decide to start leaking down and the car may go back down to the ground.
It's best to place jack stands under the vehicle when it's been lifted up by the jack.
For safety you should always place jack stands under the vehicle while it's being worked on.
Never get under a car or vehicle that is jacked up and just being held up just with a jack.
Always make sure to use jack stands for safety.
I have left a car on a jack for 6 days before and it was fine but then again I had another car go back down to the ground after a few days on the jack.
Now I make sure to place jack stands under the vehicle first and then lower the jack down.
Leaving the jack under the car holding up the weight of the car puts more stress on the jack as they are not meant to hold up the weight for too long.