Exactly, the majority of the session can last 1-2 weeks, others can drag on for a month +, its a lottery. Being away from work released on jury service for a month + can have a dramatic effect.
Once at the court, you should be asked if being placed on a long trial is going to cause any major problems for you.
When I did my service, we were told by the judge that the case is expected to last two weeks and if that caused any problems to let him know when (if) your name was called. Some people selected for the "pool" had already done a weeks service and at least one person raised this issue and was excused from the case.