To determine how long it will take for the water tank to be emptied based on the graph you mentioned, you would typically look for the point at which the line intersects the horizontal axis (the x-axis) where the water level reaches zero.
Since I can't see the image, I cannot provide a specific answer. However, if the graph indicates a water level declining to zero at approximately 11.7 hours, then the answer would be 11.7 hours.
If you have the numbers from the graph, please provide them, and I may be able to assist you further!