When trying to estimate the rate of gaining neptunium it is important to know that each of your mines acts independently.

To explain; If you have 2 mines and 12 outposts you are producing 24 neptunium per day. However, you cannot assume that you will get 1 neptunium per hour. Each mine will generate 1 neptunium every 2 hours (12 per mine per day) and this is calculated from the last time that it generate neptunium. So if you build the 2 mines at the same time then they will generate their neptunium together every 2 hours.

This can cause a little bit of confusion when the neptunium numbers in the leaderboard are changing in a rather unexpected fashion.