Probably a lot. I read a full-length book on Byzantine history, listened to another audiobook, and a few podcast episodes. (I also read a little bit about the Varangian guard, but I found myself in a hole of information that isn't super relevant to the story, so I saved the book titles to read later in my spare time.)
I've read a lot of articles about fashion, armor, and where fabrics came from. And architecture, and I've looked far and wide for good books on daily life for byzantine peasants (Had to use the interlibrary loan services so I couldn't read the whole book - I could only had it for a week!)
I like researching, it helps me get ideas! If the people living there had a specific thing they did, or found a certain ritual or omen to be important, then that's fun to use both for plot and for subtle symbolism!