Set up dbms_refresh.make interval for every 15 minutes monday-friday 5am-6pm

Hi, I would like to set up my materialized group refresh to run every 15 minutes monday - friday, 5am to 6pm.  How do I set the interval for those parameters?  Currently it just runs every 15 minutes every day.  Here's what I currently have:

BEGIN

DBMS_REFRESH.MAKE(

name => '"MIKE"."MY_REFRESH"',

list => '',

next_date => SYSDATE,

interval => '/*15:Mins*/ sysdate + 15/(60*24)',

implicit_destroy => FALSE,

lax => FALSE,

job => 0,

rollback_seg => NULL,

push_deferred_rpc => FALSE,

refresh_after_errors => FALSE,

purge_option => 1,

parallelism => 4,

heap_size => NULL);

END;

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