For automatic updating the feeds you will need to call the feedcron periodically. On unix style systems this is usually done using a cronjob. Most hosting providers allow to set up some cron.
There are two options calling the webpage using w g e t, curl or lynx. Or using the CLI version of php. The choice depends on the availability using the CLI version is highly recommend, this avoids max_execution_time problems.
a typical webpage request looks like:
0 4 * * * w g e t -q -O /dev/null http://www.example.com/administrator/components/com_datafeeds/cron/feedcron.php
0 4 * * * c u r l -silent -o /dev/null http://www.example.com/administrator/components/com_datafeeds/cron/feedcron.php
(remove the spaces in w g e t and c u r l)
a typical command line call:
0 4 * * * (cd JOOMLA-ROOT/administrator/components/com_datafeeds/cron;php feedcron.php)
JOOMLA-ROOT is the real location of you joomla installation on the harddisk. Finding the right path might be a bit of a hassle and depends on your server. Have a look in site -> global configuration of your joomla installation. Under 'system' or 'server' there are the paths to JOOMLA-ROOT/tmp and JOOMLA-ROOT/logs
each feed has it's own update frequency, ( in the feed configuration ). This is the minimal time between two updates. So if the update freqnuency is set to 360000 (100 hours) and your cron calls the feedcron every day, still the update will only occur every 4 days. If you set your update frequency to 60 seconds and your cron is called once a month the update will be once a month.