Silvery - 11:36 pm on Jun 14, 2010 (gmt 0)
You don't mention what particular data in the listing that you feel ought to be updated. If you've claimed your listing, you should go in an specify what needs to be changed/updated. If you already did that, and 6months went by without it getting updated, then you should login again to see if there are any warning notations. Most updates happen within four weeks or so -- if a requested change doesn't get updated in that timeframe, chances are good that you didn't follow Google's guidelines and they suspended the change request.
If your question is actually about how often 3rd-party data sources get refreshed, which might or might not get absorbed into your Google Place page, it varies. Different data sources for Google may update at different times and different frequencies.