Hi there,
Mines produce 1 Kilo of Neptunium per day per outpost you own. If you have 7 outposts, one of which is a mine, you will gain 7 Kilos of Neptunium per day. 24 hours / 7 Neptunium = 3.4 hours per Neptunium.
However, if you now conquer two outposts, you'll have 9 outposts, and earn 9 Neptunium per day. 24 hours / 9 Neptunium = 2.7 hours per Neptunium.
Also, this is per mine, so if you get another mine, your mining speed will double.