This is very easy just setup a single player, AI Skirmish.
I'd suggest one of the larger default planets, and adding max AI players. you need to ensure that there is a spectator slot open and you are in it. before launching turn the economy rate on all AIs down to 0.1 (meaning it takes 10x as long for them to generate resources)
In my game it was 10-15 minutes before the AI characters even had enough to build a factory. Then I went to bed, and came back in the morning and found that only 2 of the 8 factions were still alive. went to work, came home and found that I was back at the main menu and I has this and the 8 hr achievement.
Be careful, as my understanding is if you leave early(before match completion), any time in that match doesn't count toward your total