Once you unlock the CG of
, you have to view that CG for 2 consecutive hours in the Gallery Mode without quitting. You can get the achievement without waiting for 2 hours in real time by setting your PC clock to almost 2 hours ahead (e.g. 1 hour and 59 mins) and wait for several more minutes until the achievement pops up by itself.
Keep in mind that speeding up PC clock, while not considered cheating in TSA, can be considered cheating on other websites. It is highly recommended to simply idle for 2 hours instead.