It's not a perfect rotation in the sense that the same cars might appear in different order. So yes, it can definitely take 6-8 weeks for some cars.
You need these 3 cars for the trophy.
Ford Mark IV Race Car '67 (last appeared, 26.1.2023)
Ferrari 330 P4 '67 (last appeared, 3.1.2023)
Jaguar XJ13 '66 (last appeared, 11.1.2023)
You can track them
here if you don't want to check in game every day.
As for farming credits...
Le Mans WTC 700 (31 minutes on avarage) will give you
825,000 CR and
Sardegna Road Track WTC 800 (25 minutes with good pace in a Gr.2 car)
727,500 CR if you get a clean race bonus. The only time the clean race bonus for these two races is rendered invalid is when you overtake while the yellow flag is out. Otherwise you can go full rampage.
Spa Francorchamps WTC 800 (1 hour endurance) is also pretty cool and gets you
1,500,000 CR with the clean race bonus. Should be the same regarding the clean race bonus as the former two but i'm not a hundred percent sure.
Tokyo Expressway WTC 600 (around 25 minutes) will give you
825,000 CR with a clean race bonus but here you really have to be careful since its very easy to loose this bonus. I'm drawn more and more to this race lately as a diversion and it's 50/50 for me in terms of a clean race. Definitely the hardest from all of these!
And if you haven't done so already i would highly suggest you do the "The Human Comedy" endurance missions and all the circuit experiences since these are worth a lot. Not all of them are easy but especially the missions are wonderful.