I recently optimized (complete redid) a site that was getting decent rankings. However, it wasn't optimized properly and I was sure it would go up with some changes. The site was re-spidered on Saturday. However, the webmaster tools and google cache of the site just updated today (Tuesday) to reflect the new spider date. After the new spidering, the new site has the exact same ranking as the old site. This is not bad, but I was sure we would either go up or down. However, it made me wonder if google has not yet updated the actual ranking of the site. Do they do this on a regular time interval (like pagerank, but more frequent) or does this happen on a site-by-site basis shortly after re-spidering?
pagerank is normally every 3 months and site rankings in SERPs vary depending on how many backlinks you have and how much weight you pull. I usually see my index rankings change every couple weeks.