Minimum of 500 words on a page. If the page is informative and not repetitious you can make it as long as you want you'll never get penalized for adding too much good content. Some wikipedia pages for instance are extremely long.
But the reality is if you're not a site with user generated content
a) who's got that amount of time to dedicate to every single page
b) overly wordy pages on a commerce site kills conversion rates
c) chances are that much content could be broken up into a few pages which all target different keywords better