I couldn't find a "stakeholders" table, maybe because I'm still on 12.9.
However, replacing call_req for stakehodlers I was able to make the following changes to your script:
select
b.id,
c.ref_num ,
b.call_req_id,
b.action_desc,
(select DATEADD(s,b.time_stamp+19800,'1970-01-01 00:00:000')) as 'Timestamp',
((b.time_stamp - c.open_date)/3600) as Duration
from
act_log b inner join call_req c on b.call_req_id=c.persid
Since the subtraction above would get you the seconds between the activity time stamp and the time when the request/Incident/problem was opened, dividing by 3600 should get the hours you're looking for.