the more content the better, SEO-wise, but your poor visitors will dread coming back to your site if they search for something like "the best office mouse" and you start your article with meaningless shit like
What Is a Mouse
When Have Mice Been Invented
Why Are Mice Great For Computers
...
...
etc.
I mean, I would (and often do) close the entire browser instantly had I landed on such a page, and then I'd throw my computer out of the window and go play with my dog.
But google likes long articles, so you'll have to decide on your own which length you're going with