I won´t do a time estimate of how long it will take to develop this map. Am I aiming for something? Sure. I´m just not telling what that is yet.
I am still in the very early stages of learning how to make my own map for Dayz, and it would be impossible to estimate how long the learning is going to take, and even more impossible to estimate how long it will take to finish it (or even getting to an Alpha testing stage)

Realisticly somewhere between 1-2 years would probably be the timeframe. Could be more, could be less.

What I have learned so far is that the software tools used to make a map are:
1. Really hard to navigate, hard to “control”
2. Very buggy and prone to crashes. So there is a lot of babysteps followed by constant saving progress.
3. Changes changes changes. I have spent 2 days (approx 10 hours total) just placing down pine cones on the ground, in a very small patch of forest. It is very tedious work.