I'm currently playing on 90 minute days. My money making strategy early on consists of three methods:
1. Tier 1 Buried Supplies quests. They're quick, easy to find, nearby, and net you several hundred dukes per quest.
2. Looting when I'm not doing buried supplies quests and selling everything of value (armor I don't need, guns I won't use, old cash, silver/gold nuggets, etc.
3. Coolers. 8 polymers make one cooler, which is worth 60 dukes. I harvest every tire I see and turn a profit. I made and sold 20 of them on day 2 alone, netting me 1200 dukes
Honestly, making 7k in 3 days (on 90 minute days) isn't too hard; on 60 minute days, I probably would have made the same amount by day 5
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*90 minutes.

I restarted (again) after rediscovering my dislike for 120 minute days. Lol
My methods for gathering money are explained in my reply to Kosmic Kerman