I got Isaac a Doctor Who DVD for his birthday which has two documentaries on it totalling almost two hours.
Is that too much? I haven’t watched the docos yet, but it is a truly classic story (Genesis of the Daleks), with plenty to talk about in terms of how the story was written and produced, and the history of the Daleks themselves.
Memorably, the Twelve Monkeys DVD includes a movie-length documentary about the making of the film. Far from being the usual stuff about how they did the special effects, and the celebrity interviews with Bruce and Brad, it goes through the early script development, financing, and problems with final editing and test audiences.
Of course it’s all a bit over-the-top if you’re not at all interested in movie-making. Personally, I am, at least to a certain extent. So provided the extra material is well done, and not superficial vapid publicity fluff, I’ll usually take a look.