Good question, but the problem of an answer will be that the salt was the most important food-preservation method before fridges. So it is hard to say how much people ate, maybe a good starting point could be how much could they afford. And the answer is most probably: a commoner couldn't afford much.
Typically, Venetian merchants bought salt for 1 ducat a ton, and it cost them about 3 ducats a ton to ship it to Venice. There they received a State subsidy of 8 ducats a ton. The State collected a tax as the salt left Venice, and after shipping to the customer, the selling price was roughly 33 ducats a ton.
A venetian ducat contained roughly 3.5 grams of Gold which means a ton of salt cost 115,5 grams of Gold. Sadly I didn't manage to find Venetian data, but in 1300's by this source in England a typical labourer expected 2 Pounds Sterling yearly (672 gr of silver which roughly worth back in time 42 grams of gold). We can calculate that a labourer's full year salary worth 364 kg of salt.
Let's compare with the current prices, in US you can buy a kilogram of salt anywhere for less than a USD while the yearly salaries are usually over 30.000 USD
To be realistic, people could afford way less salt back in classical and medieval ages. It was more dependent on if the person lived physically closer to the source of the salt or not, or suffered heavy taxation which was implemented by French, Roman, Venetian and many other governments through history.
I would assume there are better sources than I found I am looking forward for a better answer than mine.