so mount everest weighs 357,000,000,000,000
and the average person eats 4 pounds of food per day, although you could eat just one pound a day (I don't recommend it though) if you had to
and there are 7,900,000,000 people on earth
375,000,000,000,000 divided by 7,900,000,000 is 47468.3544304
so it would take about 47468 days to run out of food if everyone was rationing and being careful, which is 6781.19349005 weeks, 1582.27848101 months, and 131.856540084 years, which is more than a century, although this is rationed out the entire time
If people were eating normally and eating 4 pounds per day, we would get 11867.0886076 days, which is 1695.29837251 weeks, 395.569620253 months, and 32.9641350211 years.
So in summary
if everyone rations out their food it will last for 132 years, but if they don't, it will only last 33