This post in this thread gives you the amount of salt in a liter of seawater. The number is 35 grams/liter.
Qadgop has given the amount of salt required per day. If your drinking water is distilled water and your emergency rations give you 480 mg then you can take in an additional 20 mg assuming no other salt intake (don’t let the sweat of your brow run down into your mouth). In that case a liter of seawater would have to be used over a period of 1750 days or 4 years, 9 mo. and 16 days.
So it’s just a wild ass guess but I would say that no survival guide would recommend mixing sea water with your fresh water to extend your supply. Better to be a little short on salt rather than long I would think since excess salt dehydrates you.