you can use a date operation (+/-) to convert this interval type to a number:
SELECT (duration + trunc(sysdate) - trunc(sysdate)) as n_duration FROM ...
or you can extract pieces and combine them:
SELECT EXTRACT(DAY FROM duration) ||
' days, ' || EXTRACT (HOUR FROM duration) ||
' hours, ' || EXTRACT (MINUTE FROM duration) ||
' minutes' as c_duration
It depends what you want to do with the interval value.