If the time is simply the number of seconds (or whatever), you could just do:
cs_id=myid | stats sum(timespent) by cs_id
But by "unix time" I guess you mean epoch time? If that's the case, I assume they must be providing a start and end time for each session, so you need to compute the duration from the difference:
cs_id=myid | eval duration = endtime-starttime | stats sum(duration) by cs_id
I really have no idea how the above information is actually represented in your logs, so it might not work (e.g., maybe the data in different events, and you have to use
transaction to construct the sessions and start and durations yourself, I don't know.