In the Game Mastering chapter, on page 506 of the Core Rulebook we get this information:
A character who successfully identifies a creature learns one of its
best-known attributes—such as a troll’s regeneration (and the fact
that it can be stopped by acid or fire) or a manticore’s tail spikes.
On a critical success, the character also learns something subtler,
like a demon’s weakness or the trigger for one of the creature’s
It is still left somewhat to GM decision what to share, but the intention is that you can learn one piece of information with one check, or two on a critical success. About what this "piece of information" is we get two other examples in the player section, on page 239:
For example, Arcana might tell you about the magical defenses of a
golem, whereas Crafting could tell you about its sturdy resistance to
These are really close to mechanical details, and I imagine this could mean a reveal of an exact numerical descriptor, such as AC or resistance, depending on play style. In most cases something like "You know it has a high Fortitude save" might suffice, as whether it has +20 or +25 might not change your decisions at all. The rules seem silent on how exact the information should be.
Also relevant is what we can read on repeated attempts to get more information, also on page 506:
After a success, further uses of Recall Knowledge can yield more
information, but you should adjust the difficulty to be higher for
each attempt. Once a character has attempted an incredibly hard check
or failed a check, further attempts are fruitless—the character has
recalled everything they know about the subject.