Salaamun Alaykum 

Thank you for your question.
The simple answer is that we do not know. 
There might have been some attempts to calculate the generations to reach a "plausible" estimation. However, the Qur'an is silent on this issue and we should be very careful not to read things "into" the Qur'an.

Essentially, the stories of the Prophets (pbut) are narrated for learning: there is moral guidance and wisdom in them. The Qur'an is generally very silent on details such as names, places and ages.

I don't want to call towards a culture of "don't ask and just accept"; rather what I am trying to say is that there is a reason why God didn't reveal certain details, and I believe the reason is to "zoom" our focus on the important parts of the stories.

In need of your prayers

Hassanain