It should take up to 10-15 minutes for it to fetch the page, as indicated in Google Webmaster Tools:
5.) Click Fetch. Google will fetch the URL you requested. It may take up
to 10 or 15 minutes for Fetch status to be updated.
6.) Once you see a Fetch status of "Successful", click Submit to Index, and then click one of the following:
- To submit the individual URL to Google's index, select URL and click Submit. You can submit up to 500 URLs a week in this way.
- To submit the URL and all pages linked from it, click URL and all linked pages. You can submit up to 10 of these requests a month
Pending that there isn't any crawling errors, it might take a few weeks for the old URL to drop off and new URL to show up, which you can check using
You should maintain the 301 redirect for quite some time after that for other search engines and incoming links.