Sterile Water is used for irrigation purposes for your health and stays sterile for up to 24 hours once opened.
Still you can use the sterile water for irrigating your nostrils even after 24 hours but it just won't be sterile like it was before it was opened.
It's best to use the sterile water within 24 hours but always close the bottle of sterile water right after you use it to preserve the sterile water until next use.
When not in use store the sterile water inside the fridge once you open it to keep it sterile.