That will depend on how often the Googlebot visits your site.
Even if the ranking of the page is "improved" (note I didn't say "page rank") it might not change the actual position in the SERPS for a particular keyword. You might go from being only just number 10 to almost but not quite number 9.
Thanks for the reply, to give a little more information we currently rank 67 for a particular search, I have made some changes to the target page and wondered how long I should wait to see if the changes have made a difference. The page has been submitted to google index via Webmaster tools, searching for 'site:domainname page' shows the new page title and meta description, so I know that google have found and seen the updated page.
Wait a bit longer and then check. But it may be what piatkow said - that despite changes you will not move. Or you may be moving down or may be "dancing" for a while.
I would also suggest that you read this thread: Google's Rank Modifying Patent for Spam Detection [webmasterworld.com], there is a speculation whether this is actually used or not in the algo, but it is worth reading through, especially if you are doing changes to the page "for ranking purpose only".