How long does it take to propagate a SEO change?

Loaded question with many variable. Hours to weeks depending on the overall site value with Google specifically. Higher page rank etc tends to see faster turn around.

If you are in need to decrease the time, i would suggest implementing feedburner to the site. I find that it greatly improves the time from change to listing.

Andrea,
The frequency with which Google crawls a site has to do with a number of factors, including its popularity and its history of updates. Some sites are crawled numerous times per day, others maybe once per month. The way to tell if it has crawled the site is to do a site:domain search in google and review the search results.

So, for example, go to google and do this search: site:yourdomain.com (using your domain name) and you will get a list of pages for your domain. If you made changes to your meta data on a page and Google's recrawled that page, you'll be able to see the revised title / description in the snippet for that page. If you didn't make changes to the meta data and only changed the contents of the page, then when looking at the results of your site:domain search, click on the little down arrow to the right of the green URL that shows in the snippet and click on "cache". That will show you what the page looked like the last time google crawled it. From that you can tell if Google's seen your changes. Note: it can take a day or so after crawling for google to make adjustments to rankings if it's required.

If you have a Google Web Master Tools account for the site.

Use the Fetch as Google Tool. After you fetch the content just click the submit to index button.

Google wills see your new stuff no problemo.

I use this when I build a new page or make considerable changes to an old one. Typically 24 hrs after publishing a new page I can search the key phrase it's designed around and see it appearing in the returns. Sometimes it only takes an hour or less to see it appearing.

It all depends on how often the search engine spiders come by your site to get content updates. How often this happens will depend on your page rank and authority.

It could take weeks or months. No one has an accurate answer for this.

I hope this helps!

Cheers!

When I usually make a website for my client... It takes around 8 -12 weeks to index the site... but I optimize it when I am making it... don't know how will it take if you "re-optimize" the site...