You an change the default on the instance using the info in this post:
http://answers.splunk.com/answers/106136/how-to-set-the-default-search-time-in-splunk-6
In ~/etc/system/local/ui-prefs.conf on your search head, set the following which will set "Last 15 minutes" as your default search time range in the picker:
[default]
dispatch.earliest_time = -15m
dispatch.latest_time = now
You an change the default on the instance using the info in this post:
http://answers.splunk.com/answers/106136/how-to-set-the-default-search-time-in-splunk-6