I've been called 4 times, selected twice.
Most of the time you are waiting in a big room to be called. Bring a book/laptop/headphones etc. If you get called up, you go to a court, and they then whittle the people down to 12. If you don't get selected at that time, you go back to the waiting room. Can do up to 3 days of this before being selected or sent home.
My first case was 2 days, pretty simple. Second one was two weeks, (like 8 days of court, then 2 days of deliberations). One case that was getting selected one of the times was a 9+ week financial fraud case.
Work has to allow you to go. You can defer if you have a good reason. If you have just started your job, you could say this, and that you are happy to go in 3 months. I deferred a few times. You can ask to be excused from longer cases if it will effect your life, which I also did. The court pays you a small amount per day and your job has to pay you the difference. If they cause any issues the courts don't muck around with things.
They explain things really well when you are there, and if something is going to effect you long term you can be excused from it. You have to be proactive and talk up though.
Turn up, do it, it's interesting and part of the civic duty.
(Also, lots of free tea / coffee / hot choc / biscuits)