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During a recent ovehaul of my site (over a week), G just happened to index the site during updates.
I returned the next day and discovered additional updates (due to my updating...)...and crawled the site again..
My best guess it it thought that the site was being updated very frequently during that week, and ended up coming every 20 minutes or so looking for updates..
since the update, it's slacked off to once a day (or so)...however I make small updates every couple of days to keep it fresh, and entice indexing of my site.
This also holds true to web sites that are never updated, I believe G measures time/updates to compute how often to re-index the site. If you've never changed your site in 24 months, don't expect the crawler to be there anytime soon...it has millions of other sites that ARE updated regularly.
If it's a new site, you probably have no control over when, but once you see that it's indexed a page or 2 do a small amount of changing to the pages so that it at least thinks the site is kept fresh. I believe that will help in the long run.