“…it would indeed appear that one anantakappa corresponds to twenty billion years. I now hope I have got this right myself!!”
See? I did NOT get it right. Any of the four phases lasts twenty anantakappa (twenty billion years); so,in reply to the question, one anantakappa would equate with ONE billion years.
We do not need to give overdue importance to the actual lenght. It cannot be changed, whether we find out how long it is exactly or not, whether we think about it or not. What we CAN change is our destiny.
What is important is to realize that we have to do what is necessary NOW to get onto and to advance on the Path, during that ‘duration’ phase, however long it is in actual fact.