Learning Dhamma in a small way
Probability of becoming a Buddha by a human.
Here it is a good idea to consider what the Buddhhood is and how difficult and the long process it is to become a Buddha by a human being. It will always be a human who will become a Buddha according to Lord Buddha's discourses.
One needs to have a good understanding of Time to grasp the foregoing descriptions so let us do some homework first. How do you explain Time or how do you understand Time yourself ? We can say that it is a personal experience of a human living in this world, who considers the time duration of a Sunrise to the next Sunrise as a day. We know that the length of a day in our Solar System's planets are vastly different to the time duration of the planet Earth's day. Suppose a child born in our Earth was taken to , lets say Jupiter the moment he was born and was able to live in that Planet, and when he is able to understand and think as a normal human, he also might consider a day from a Sunrise to the next Sunrise and call it a Day. Invariably the so called durations of the days here and there will be different. What he will call a Year and an Year here will definitely be different. So ultimately, the thing we call time is a conceptual thing in our mind and the biological system. The point wanted to make here was that what we call a year's time or ten years time or hundred years time will not be a definite measure but only a relative thing for us humans of this world. When we call the life span of a man , it will mostly be below 100 years. On civilizations, we call them to be many thousand years old. The age of the planet Earth and the Solar system will go into Billions of years back as scientists say. So those values into millions and billions of years are only numerical values for us humans. Now lets see how often a Buddhahood is attained among human beings. According to the Gautama Buddha's discourse, that time will be in terms of ' Kalpa '. The time duration of a Kalpa is unimaginable and needs to be understood by analogues , or comparison, the Buddha said.
The First comparison of Kalpa or Aeon is,
There is a cube by the size of 7 miles in Length, 7 miles in Breath and 7 miles in Height. which is entirely filled by tiny mustard seed. A person removes one mustered seed once in 100 years. From this process, the cube gets empty with mustered seed after a long time. Buddha says that a Kalpa is even longer than the time which took to empty the cube with mustered seed.
This comparison can be put into numerical values for an easier understanding on how great a period of a Kalpa is.
If the size of a mustard seed is taken as 3 mm, then to complete this process would take 936 quintillion years or 38 billion times the age of the universe.
 Mustard Seed From Wikipedia, the free encyclopedia [link]
the Second analogue is,
There is a Rock of the size of 7 miles long, 7 miles Breath and 7 miles Height. A person wipes the rock with a Silk cloth once in 100 years. Doing so there is a time that the Rock disappears due to this process. Buddha says that a Kalpa is even longer than the time which took the Rock to disappear by the above process.
So what made Buddha to put forward such analogues in his disclosures. That is we humans will be at a loss to understand the unimaginable dimension of Time by our knowledge, since the age a human living in this world is just a speck on the unimaginable Timeline.
So having heard of how lengthy a period of Kalpa, he has mentioned about Six Buddhas prior to him who disclosed Dhamma, or the Noble Truths after attaining Buddhahood.
Vipassi Buddha - 91 Kalpa prior to Gautama Buddha
Sikhi Buddha - 31 Kalpa prior to Gautama Buddha
Vessabhu Buddha - 31 Kalpa prior to Gautama Buddha
Kakusanda Buddha - The present Kalpa
Konagamana Buddha - The present Kalpa
Kashyapa Buddha- The present Kalpa
Gautamai Buddha - The present Kalpa
So A Buddha appearing so seldom has an unimaginable mental factors that were accumulated by refinement throughout Samsara or the continuous cycle of Birth, Life and Death , prior to the Buddhahood.