I wrestle with this question all the time.
As
a player I'm definitely part of the subgroup that wants to dive in and tease apart all the nooks and crannies of the world of Arcanis ... because that's my style of role playing AND because Henry plants plenty of Easter Eggs in the campaign. As a result I like to think I know as much as anyone in the player base about *what is really* going on.

That means regardless of what character I'm playing I often have some idea of what I'm looking at unless its completely new. I try not to metagame IC too much. It helps me to have a clear idea of what my character knows.
How much do characters know in general?
Really depends on the character. Thrak the illiterate dark-kin gladiator with a spike chain probably knows nothing.
"Oh, my GOD - Bug Men! That's not right, Run!" One the other hand, my main character Tukufu is from Altheria (known for scholarship), has some ranks in Knowledge History & some more in Knowledge Myths and Legends (among others), is a member of the Emerald Society (shhh don't tell), got a packet of secret knowledge as part of character creation (special reward character for writing modules in the IK program), is psionically awakened (and thus has studied the arcanum), and finally "officially" owns a book store in the First City with a nifty rare books room (via Blessed Lands Kickstarter). Put that all together and its my conceit that Tukufu knows (or suspects) most of what I know as a player.
"Oh my god! Bug Men! Quick, someone count the legs! Are these Il'Huan or Issori?"I personally believe that if you have the right kind of player character concept, and have taken time to invest in supporting mechanics (in this case predominantly the three knowledge skills of History, Myths and Religion) then its reasonable to assume you have *some idea* of the weird stuff other adventurers have wrestled with in the past. "Bug Men! Hey they might be the same as the Il'Huan who attacked Enpyben!"
How much is some idea? I have some guidelines I use with Tukufu.
ONE: I try to stay out of mechanics. Tukufu never talks in game (or in his letters) about "what the AR of something is." He might say something like, "I'm guessing, based on what was written after Enpyben, that the carapace of an il'Huan is better than Lorica. But I've never had the chance to study one personally. They may be extinct again...certainly I haven't heard of a sighting since the Coryani Civil War."
TWO: Conflicting Tales. If an old module had more than one possible outcome I assume that Tukufu (if he has heard of it) has heard conflicting versions of the tales where both (all) outcomes come to pass. Unless its empirically obvious what outcome is correct I assume that has no reconciled the sources. Where appropriate to comment in game, I deliberately try to offer multiple interpretations of "facts." In the case of the The Hara’mia I assume Tukufu has gotten a couple of different tales. He was freed. No he wasn't freed. He spoke a bunch of lies. It was a prophesy. No he spoke of temporal paradoxes. And Tukufu doesn't really doesn't know what to believe about what he's heard. On its face the story of that module IS pretty outlandish and takes place in the middle of an important rescue mission and the assassination of Menesis. Sort of major events in their own right. So maybe at the time important people weren't all that worried about the Hara'mia.
THREE: Some "knowledge" is being actively suppressed. If you don't have a reason to be in on the secret then that PC (IMO) should be ignorant of it. Take "the Silence." Tukufu is a psion who has also been at four "special events" where "silence-y" entities were around. As a result I play Tukufu as aware (and somewhat afraid) of the Silence. As a counter example Tukufu really has/had nothing to do with the val'Inares family or the Temple of Anhar. So I play Tukufu as completely clueless about the connection between Anshar and Yig. You'll never see him write a letter on the subject unless he somehow comes upon that knowledge in game and its appropriate for him to share.
LAST: I try to play nice. That means trying to avoid spoilers, or stealing the thunder from any other PCs, or writing about things before "their time." For example while Henry was at Gamestorm he shared with me what the
Gift was that Beltine gave to humanity at the dawn of the First Imperium. It's creepy and wondrous, as befits the Goddess of the Cauldron. It seems likely to me that Tukufu knows of it so I asked once if I could share it in one of Tukufu's Letters (as we got close to Halloween) and Henry asked me not to. So I'm sitting on it. It could be part of a future module ... or it just might not be the right time to share yet. I think Tukufu knows, but I want to be careful with it as a responsible (and respectful) member of the campaign.