There isn't a formal definition of success.

Intuitive seeming is in my view an attraction to assent based on nothing more than understanding of the content that upon consideration attracts one's assent. But such understanding-based attraction can differ dramatically in epistemic quality. Some such attractions represent nothing more than superstition or bias absorbed from the culture, sans ratiocination.

We can pursue the Cartesian project without restricting ourselves to theology and a priori faculties. A better, broader perspective is properly sought if we pursue the project with reliance on science broadly and on our full span of epistemic competences, including the empirical as well as the a priori.

Compare the credit for a football touchdown, which might be shared by the receiver not only with the quarterback, but also with the linesmen who make crucial protective plays, etc. The success of the touchdown play depends on the receiver, it is true; but in a particular case it might depend far more on the work of others.

A successful account enables us to understand human knowledge in general.

Epistemology now flourishes with various complementary approaches. This includes formal epistemology, experimental philosophy, cognitive science and psychology, including relevant brain science, and other philosophical subfields, such as metaphysics, action theory, language, and mind. It is not as though all questions of armchair, traditional epistemology are already settled conclusively, with unanimity or even consensus. We still need to reason our way together to a better view of those issues.

The success of an archery shot may bring food to the hunter's starving family, or may constitute a horrible murder. But these outcomes are irrelevant to the assessment of that shot as a hunter-archery shot, as an attempt to hit prey without running excessive risk of failure.