There are various opinions about when to slaughter a rabbit.  Of course, you want the most meat you can get, but with a Flemish Giant, they grow until they are 18 months old.  Even though the rabbit could weigh as much as 22 pounds, waiting that long, would yield tougher meat and cost you nearly 400 pounds of food for this "little guy".  That's a very expensive and not very delicious meal.  Clearly, a younger butchering age is in order. 

Obviously, when it comes to "fryers", the most tender meat is going to be found in a young rabbit, but don't butcher it too young.  Flemish Giants, in particular tend to grow a lot of bone at the begin of their lives and add meat quickly after about 4 weeks of age.  The difference in size between a 4 week old and an 8 week old is striking.  The trick is to catch the rabbit at just the right time when it has emassed the most meat possible for the least amount of time/feed. 

In our opinion, the tenderest meat, while still being the most cost effective is between 8 to 10 weeks of age. At that time the rabbit should be between six to 10 pounds which would yield you roughly 3 to 5 pounds of boneless meat.

For an older rabbit, you could always send it to the stew pot rather than frying and the difference in tenderness would not be as much of a factor.  I would still suggest that the rabbit around 6 months old.

Those who raise other breeds may balk at the timing I have suggested, but remember that Flemish Giants get a lot bigger than other breeds and that is why many people choose them for their meat rabbits.  They are ready at a younger age and the wait and the cost of feed is not as great as it would be with other breeds.