It really depends. For instance, most magic items shouldn't take more than 200 words to explain what they do, unless the item has tons of different abilities, but the history of an item could perhaps take two or three pages on its own. It depends on whether you just want to have a book with stats for mythic items, or if you want a book that captures the feel of these mythic items.
I personally would, if I were writing such a book, probably give each item two or three pages (1000 to 1500 words). One page would be the real item's myth, including some prominent re-imaginings that have cropped up in pop culture. One page would be other twists on the myth to make the item fit into different styles of games, and to just be cool. Then the last page would be stats for different power levels of the item, and possibly an adventure hook or two.
Honestly, I think it's too cool of a topic to short change. While it'd be possible to simply have 30 items with two-paragraph write-ups, it wouldn't feel right.
