g1smd - 6:26 pm on Nov 15, 2012 (gmt 0)
I see a big spike on most sites at least every 6 to 10 weeks. Very often much of it is Google revisiting pages that previously returned 404 or 301 and which they then scheduled as low crawl priority.
404 pages are checked a couple of times within 48 hours of being discovered as no longer returning 200, then Google doesn't come back to that URL for weeks or months. This means that you need to be VERY quick to fix unexpected 404 errors on your site otherwise the corrections are not seen by Google for a very long time.