When and how often does Google index?

The question of when and how often Google indexes has been debated in SEO forums and blogs all over the world. The best answer I can find is this:

Once you have received an initial indexing by Google (for info on how to do this, read this Google article) Google will reindex your site every 15-30 days. However Google also can detect when a site is updated often. Some forums and blogs are reindexed as often as daily or every other day.