Getting to the caves from "downtown" is a 30-45 minute walk up a hill. When you get there you buy a ticket for a tour that starts at a specific time. Depending on how many people are already there you will have to wait 15, 30, minutes, an hour perhaps for your tour to start. In the mean time you can look at an exhibit they have and tour the grounds. The tour itself is about 30 minutes. So plan 3 hours.
Is it worth it? Depends. It's not something visually extraordinary, but it is an important part of our history and interesting from that perspective. The explanations on the different painting techniques are also interesting. Can you see the real thing? Only if you are lucky. The blanket answer is no, only the replica, but every Friday morning there is a draw amongst the people there and 4 of them, I think it's 4, are allowed in the real cave. I have heard that their are disscussions at the moment to increase access to the cave in the future as it is no longer clear that human presence causes deterioration, but that is not for tomorrow. Hope this helps.