If the migration is done perfectly, both domains are already very strong, all content is migrated (and no new keyword cannibalization issues are created), the move should be recognized in a matter of days.
Things to consider:
1. How does the to-be-moved site get crawled and how deeply? A key to successfully moving a site is ensuring that the search engines quickly find all of those 301 redirects.
2. XML Sitemaps can help with a speedy transition. Here is the trick, though. Be sure you have an up-to-date XML Sitemap of the to-be-moved domain just before the transition. Once the move is live, upload the old sitemap to webmaster tools to help get Google looking at those redirected pages.
3. SEO is a relative excercise. There are a lot of variables at play here is both domains show up in the SERPs for the same important keywords. Expect a lot of change as a result of the move. Look out for keyword cannibalization.
I hope this helps!
Lindsay
