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.