Purely going on being optimal with computer resources, running a highly physics game with a strong video card to read static text is... kinda meh.
Recently, with Batman Arkham asylum, Metal Gear Solid finding tapes and listening to interviews, missions and things while you're exploring and doing stuff that doesn't require character interaction seems more optimal than sitting and reading, unless you have to. This isn't games where the CPU runs half the video output so static text and low power usage is relevant here.
If i can listen to lore and story while dungeon crawling, yes please i'll take that. But unless books/notes are particularly engrossing, i'll skip reading them. Sure Morrowind and Oblivion had some funny stories, but so much of it is dry that i collect books to collect the series, not to read them. I'd more enjoy collecting books, and then when i'm NOT playing on my computer, say i'm on my phone/ereader on a drive, i could open the collected books and read through them as PDF's and get my lore that way, but not in game, feels like a waste.
So... as for actual percentages. I'd say 5 minutes of cutscenes and reading text, to say an hour gameplay. And let's assume another 5-10 minutes character interaction for quests. That's about 15/85.