When I wait for your reply I was still search answer for my question. And I found some information... (from end of 2005 yr.)
Well, its not an officially decalred! but google also consider aging factor, okay for the timing keep it in side. Google crawled each and every page on your website which a bot can see, and stored them into repositry. Now for the timing Google indexing frequency depends on PR, AGE, EXTRAPOLATION ALGORITHM( use to calculate how often, this page will change in future.) But Google said "we fully travese our repository once in a pre-defined time." which is about 60 days!
By the way, now google is maintaining a supplemetry index, which is probably bigger than active index.