Most of the times we need to configure a timeout for all the ajax calls fired from anywhere in the sencha application. This can be done by simply setting up some properties for the Ext.Ajax, Ext.data.proxy and Ext.data.connection classes during application startup.
You may add a function setup to Application through Sencha Architect just to keep this code in a separate function (otherwise you may add the code to launch method as well. Below is the configuration required for setting global default ajax timeouts
Above code sets the timeout to 300000 ms and additionally disables the default xhr headers to false (disabling this might save you some time in case you face any issues when the headers provided to your ajax calls seems to be changed)
Edit 05-01-2017: For Extjs 6.2 just adding below to launch method should handle the default timeout for ajax in your application :