One of the main features to configure while developing a website that has access for users (in frontend, backend or both) is to set the session timeout. Its value defines time (measured in seconds) after which user session data will become out of date, if that user has performed no activity. For example, the default session expire time in symfony is 1800, which is 30 minutes (60*30), if a user is logged in and he won't do anything for at least 1800 seconds, he'll be automatically logged out.
There are several blog posts in the internet describing where to change this value, but they refer only to symfony 1.0 (which is really old... and solutions described there do not apply to sf 1.3/1.4). So, the file is apps/APP_NAME/config/factories.yml. You need to add the following lines:
all: user: class: myUser param: timeout: 7200
where 7200 can be replaced with whatever unsigned integer value you want. Take a look at the main factories configuration file in symfony core: lib/vendor/symfony/lib/config/config/factories.yml. You'll find the default/user section there which holds all interesting stuff.