On a per user basis you can go to the Options area (Logged in as <user> -> settings -> options) and change it there, Globally you need to add the setting to RT_SiteConfig the perldoc for RT_config says:
$HomePageRefreshInterval $HomePageRefreshInterval is default number of seconds to refresh the RT home page. Choose from [0, 120, 300, 600, 1200, 3600, 7200]. Set ($HomePageRefreshInterval, 300); #5 min refresh by default Chris
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