Well, with the latest theory on interstellar travel, this would take, at the fastest possible speed, 8 years round-trip. This isn't including landings, time spent mining, or launchings. However, if we were to mine this planet, diamond would suddenly be worth less than dirt.
Reason:
1. Faster than light travel:
http://techland.time.com/2012/09/19/nasa-actually-working-on-faster-than-light-warp-drive/2. Prices change based on supply and demand, a system that requires a scarcity of goods, and this planet is made of almost solid diamond and twice the size of Earth. This means there will be more diamond available to us than all the dirt on the surface of the Earth, and let's face it, that's what we have the most of.