alter session set nls_date_format= 'dd.mm.yyyy hh24:mi'

 alter session set succeeded.

with t as (select 45678 device_id, to_date('12.12.2015 15:21') op_dt, 1 on_off
from dual union all select 45678 , to_date('04.03.2016 03:23') op_dt, 0 
from dual union all select 45678 , to_date('07.04.2016 13:43') op_dt, 1 
from dual)
select device_id
     , sum(round((op_end_dt - op_dt)*86400)) "on_duration (seconds)"
     , numtodsinterval(sum(op_end_dt - op_dt), 'day') "on_interval (days hr:mi:ss)"
  from (
    select t.*
         , lead(op_dt, 1, sysdate) over(partition by device_id order by op_dt) -1/86400 op_end_dt
      from t) t 
where on_off = 1
group by device_id

DEVICE_ID                               on_duration (seconds)                   on_interval (days hr:mi:ss) 
--------------------------------------- --------------------------------------- --------------------------- 
45678                                   20259623                                234 11:40:23.0              

1 rows selected