Subtract two dates stored as varchar
Here is the sample query to convert the given varchar2 value to date and calculated the difference in hour.

select (to_date('20160316124052', 'yyyy-mm-dd hh24:mi:ss')-to_date('20160316080002', 'yyyy-mm-dd hh24:mi:ss'))*24 DateAndTime from dual;


Update

SELECT (sysdate-to_date(START_TIME,'YYYYMMDDHH24MISS'))*24 FROM LOGGING WHERE NAME='TERR_PAST' AND UPPER(STATUS) = 'EXECUTED';