I see no reason to put limits on these things. Old enough to understand it, obviously. Apart from that, any age should be fine. As for the second question, so long that you've got everything you needed or wanted from it, got completely sick of it, and then got all the subtleties that you didn't know you needed or wanted. And then some more. That's how long I think you should study it, though I, for one, have not even tried to do that. But I think the answer to both of these questions varies greatly depending on the person reading the book. And even then, I don't think a limit needs to be set.