To know how long in seconds one collision can happen, it is only necessary to take the inverse leadings to:
2.08 1014  secondes,
the equivalent   of 6 959 560 years.
As this value is huge, we can't give the result as
a frequency: 1 collision every n seconds. 
From a statistic point of view this collision can
happen at the beginning or at the end of this calculated time,
then it is necessary to divide by 2 this value for having the average
for several runs. 
