Usually just a few hours.
As you maybe aware, MacRumors usually provides a spoiler free link before the keynote, which can be bookmarked by anyone wishing to view the keynote without any knowledge of any released products. It essentially provides a direct link to the Keynote stream, allowing the user to bypass Apple's homepage, along with the MacRumors homepage, which will both contain information about released products.
I'm sure more information about MacRumor's coverage of the keynote will be upcoming as the keynote approaches.