I was out of Japan, traveled for 18 months, in the past 3 years.
Suppose I die at the age of 82. I have 50 more years to go. If I divide this length of time by the number of countries in this world, it is approximately 3 months per country.
That means, by the theory, I can visit every single country before I die, if I live long enough and spend good amount of time and money for traveling.