one 劫 is how long?
there are small, medium and great kalpa 劫.
The length of the process on the level of a world system is
called a great aeon/kalpa/劫, or mahakalpa in
Sanskrit. The length of a mahakalpa is calculated as follows:
"Starting from a lifespan of ten years, for every hundred years the
age of people increases by one year, and their height increases by
one inch. This keeps on increasing until the lifespan of humans
reaches a full 84,000 years. Then this is followed by a process of
decrease in the same ratio. For every hundred years, there is a
decrease of a year and an inch from the lifespan and the height of
a human being, until his age reaches ten years again. One complete
process of increase and decrease makes up one kalpa--16,798,000
years. A thousand of these make up on small kalpa. Twenty small
kalpas make up one medium-sized kalpa. Four medium-sized kalpas
make up one great kalpa (1,343,800,000,000 years). Each of the four
stages takes up twenty small kalpas--twenty kalpas for coming into
being, twenty kalpas for dwelling, twenty kalpas for decaying, and
twenty kalpas for going empty." (EDR I)
http://online.sfsu.edu/~rone/Buddhism/BuddhistDict/BDT.html#time
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