I have visited the Chamarel Rhumerie more times than anywhere else on the island and in most cases I have taken friends and family there for lunch.
The guided tour takes about 15 - 20 minutes for which there is a small charge. Once over you will be offered rum tasting (within the entrance fee) which consists of around 9 different rums. Having lived on the island now for a number of years I have become a rum fan and a fan of the rum produced by the Chamarel Rhumerie, even to the extent I am the proud owner of a bottle of their 10-year-old rum, number 66 of 500.
Here's a tip for you, if you are going to visit, have lunch (see my review in the 'Top 10 places to eat in Mauritius'), and if you book lunch the tour is free! So, you get the tour, you taste the rum, you have a great lunch and as a bonus, they have a great gift shop so you can take some rum home with you. Whats not to like!