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Go to the web.config file at the root level:
To increase the IIS Connection timeout value, change the following value
add name="CMSConnectionString" connectionString="Persist Security Info=False;database=xxx;server=xxx;user id=xx;password=xxx;Current Language=English;Connection Timeout=<Increase this value>;" />

OR
Change the below: 
<system.web>
    <httpRuntime maxRequestLength="51200" requestValidationMode="2.0"/>
to
<system.web>
    <httpRuntime executionTimeout="180" maxRequestLength="51200" requestValidationMode="2.0"/>

This will increase it to 3 minutes.
http://devnet.kentico.com/Blogs/Juraj-Ondrus/May-2009/Few-hints-for-large-file-uploads,-authentication.aspx

Also look at the below setting (Very important Setting)
http://devnet.kentico.com/docs/devguide/index.html?staging_large_files.htm

Adjusting File Size Limit in IIS 7
The problem is that in IIS 7 on Windows 2008 Server, the web application will reject any file that is larger than 30 MB. This is a default limitation of IIS. You can increase the maximum file size by adding the following code to <system.webServer> in the web.config file:
<security>
 <requestFiltering>
  <requestLimits maxAllowedContentLength=”2000000000″ />
 </requestFiltering>
</security>