As they say SharePoint Lists supports Items upto 50 million - During read operations.
However the challenge has always been to make the data in the large list available for retrieval and display in timely and efficient manner.
Although the users can create as many items as they want the query operation shows down as the list grows in size.
So what do poor administrators like you and me do ...
Introducing our life saver : Resource Throttling
Resource Throttling is new with SharePoint 2010 and is set on site collections by default standard users are throttled when retrieving 5000 items from the list with a warning of 3000, these warnings can be changed using PowerShell.
the mechanics works like this:
if a user is pulling data with 4999 items the query runs all the data without any error.
however if the user or query tries to retrieve 5001 items an expensive query will be returned.
so what we can do is we can use Central Admin to set a time frame when these expensive queries cannot be run for example a query cannot be run at some time frame like 9 to 6 and then later these query would yeild results.
this can be done from Central admin>Application Management > Manage web application > select your application> General settings > Resource Throttling.
hope this would help :)