The short answer is about two weeks. But there is more you can do if you know the car won’t be driven for a while. You can prevent the battery from dying at all, and you will avoid reducing the life of the battery in the process.
Start the car up once a week no matter what. This will recharge the battery. Keep it running for about 15 minutes. This lets the alternator recharge the battery and keeps the engine in good working order, too.
Turn off the lights (if at all possible) while the car is in that 15-minute period. If you don’t, you may experience a “deep discharge” the next time you try to drive the car. Lead-acid batteries aren’t designed to handle this process, especially if it happens multiple times. The lifespan of the battery will be reduced by at least a third.