Most modern people would die in less than 40 days without eating anything.

Maybe a bit longer if they took vitamin supplements.

I'm sure the grass would extend that time by couple of days if they would consistently eat grass.

But...

Grass doesn't taste very good, it's not pleasant to eat, and even if you did have good tasting pleasant to eat food, if it was the only food you were allowed to eat until just before you die from malnutrition you'd go bonkers.

I think it takes like 2-3 months to actually die when you don’t get any calouries

There's a guy in Scotland who went for over a year with essentially no caloric intake.

https://en.wikipedia.org/wiki/Angus_Barbieri%27s_fast

Angus Barbieri (1938 or 1939 – 7 September 1990) was a Scottish man who fasted for 382 days,[2] from 14 June 1965 to 30 June 1966.

For 382 days, from 14 June 1965 through 30 June 1966, he consumed only vitamins, electrolytes, an unspecified amount of yeast (a source of all essential amino acids) and zero-calorie beverages such as tea, coffee, and sparkling water, although he occasionally added milk and/or sugar to the beverages, especially during the final weeks of the fast.

After 1 year and 26 days without food, Angus Barbieri ate his first solid meal at 10 AM on 11 July 1966: a boiled egg and a slice of buttered bread.[5] He said to the gathered newspaper reporters and photographers, "I have forgotten what food tasted like ... It went down OK. I feel a bit full but I thoroughly enjoyed it."[4]

That being said, he had a lot of body fat on him going into the thing, which he lost; it was his body consuming that fat that provided the calories.

Barbieri went from almost 33 stone / 456 pounds (207 kg) to 13 stone / 180 pounds (82 kg), losing 20 stone / 276 pounds (125 kg)

Most people aren't going to have enough fat to go for over a year.