In an ambulance or in the hospital, they shove this bag of salt into you easily in a day.
Doctors inject you with this saline solution when they consider that there is a risk of dehydration or electrolyte imbalance in your body. For this reason, they add a higher amount at that precise moment.
So why do they say to take a max of only 1,500 mg when you are on your own?
Because more can be harmful to health, especially by raising blood pressure. But this is by habit and in the long term... not by a bag of saline in a moment of illness or accident.