This is self explanatory. When you are done with everything, you may only be on 30-35 hours. After this, you can keep playing, or just leave the game running. You don't have to be actively moving. The game will occasionally show the progress towards it, and you need to rack up 2400 minutes. This carries over between playthroughs, so if you prefer you can start a new game to make up any remaining time. In addition, time played in the DLC does count towards this too, so you can move on to the DLC instead of idling for this trophy after doing everything else.