I believe this would create a bad incentives structure. You'd penalize the author for people getting hooked to the book (and therefore getting into the 'flow' and reading faster), and encourage scammers to just linger on pages (probably making the scam even easier).

Plus, lots of significant ambiguities to solve: user is reading a page, gets up to do something else, forgets Kindle open. How many minutes do you bill? This might be solvable with the proper signals and rules, but I believe this is far from trivial.