Time difference being in days, let us multiply it by no of hours 24 and no of minutes 60. Assuming they are of DATE
or TIMESTAMP
datatypes
select (
to_date('01-01-2014 '|| end_time ||'00','DD-MM-YYYY HHMISS') -
to_date('01-01-2014 '|| start_time||'00','DD-MM-YYYY HHMISS')
) * 24 * 60 as time_elapsed_in_mins
from your_table;