I decided to test the myth that salt can be used for fixing photos. I started with a concentrated solution at normal temperature and I decided to wait as much as possible. Well, after 24 hours the film was quite fixed, i.e., clear and resistant to light. I did not test for residual silver, but I putted the negative at my living room window and it remains unchanged for 10 days now. No signs of deterioration. Besides I putted two negatives but one fixed with ammonium thiosulfate to serve as comparison.
I repeated the experience with more concentrated solution, until I reached the value of 1,225 g/l. And the duration dropped from 24 hours to just 3 hours at 28ºC. Well, I can now estimate a graphic like this:
This graphic is valid for the kitchen salt I am using (bought at LIDL) and for the film Fomapan 100 ASA only. Tap water from Leiria, Portugal.