SQL - Find cycle and duration

I search a solution for my problem in SQL (postgresql).

I have a logs table with the following columns:

I search one query (or more) to find the time elapsed between two codes (8 and 9 for example).
The output of SQL query will be:

I would really appreciate if anyone can suggest some ideas how to solve this problem.

I tried with lead function but i can't reset time between each sequence (8 and 9) and ignore unique 8 or 9. In short, i'm stuck and I don't know if it's possible in SQL.

This can be accomplished by using the window function lag() to obtain the previous row of the current row, followed by subtraction to obtain the elapsed time between 8 and 9 using the condition where code = 9 and lag_code = 8:

 with cte as (
  select *, 
            lag(timestamp) over (partition by deviceid order by timestamp) as lag_timestamp,
            lag(code) over (partition by deviceid order by timestamp) as lag_code
  from mytable
  where code between 8 and 9
)
select deviceId, timestamp - lag_timestamp as elapsed_time
from cte
where code = 9 and lag_code = 8

Result :

Demo here

Use this simple query to produce the result you need (and test it on fiddle) :

select a.deviceId, (b.timestamp - a.timestamp) as elapsed_time
from logs a join logs b
on b.deviceid = a.deviceid
and b.id = (select min(id)
            from logs b
            where b.deviceid = a.deviceid
            and b.id > a.id
            and code = 9)
where a.code = 8;

and it gives:

 deviceid | elapsed_time 
----------+--------------
 device1  |            2
 device1  |            1
 device2  |           10
 device2  |           30