@samlinton You’re right, there are factors like thickness and cuts of meat that determine the cook time ranges. For example, ribeye may be cooked from anywhere between 1-4 hours.
A general rule of thumb is that longer cooker time will get you more tender results. Serious Eats has a really great guide on sous vide steaks. Talks about everything you’re asking here. There’s also a basic time + temp explanation in the community.
You can cook fatty steaks - ribeye is one of them. What was the outcome of the fatty piece of beef you cooked up? What kind cut of beef?