If there's a dramatic change it may make dramatic changes to rank, subtle change may move the site a bit, ideally you have regular updates to the site happening so that the site climbs bit by bit.

I find using the google site map ensures my site is regularly checked for updates, and they tend to be reflected in the rankings within a week of the last scan, often much sooner.

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Ron Boyd
website consulting (design, optimization, marketing) :: Follow Me: @orionsweb