If you're planning on leaving your car on jack stands for an extended period of time, there are a few things you need to keep in mind. First, make sure that the jack stands are rated for the weight of your car. Second, check that the stands are stable and level before getting under the car. Finally, use chocks to block the wheels from rolling. Assuming that you have a properly rated set of jack stands and that they're stable and level, you can leave your car on them for as long as you need to. Just be sure to check on the car periodically to make sure that everything is still in place. And when you're ready to move the car, be sure to lower it slowly and carefully to avoid damaging the stands.