The Cornish Cross is considered to be finished at 6 to 8 weeks old when commercially raised, at 8 weeks they are so heavy that their legs break along with other bones. If you slow feed them and let them free range they will put on weight much slower than if given access to feed, but I personally would not have them for either meat (what they are bred for) or eggs. There are so many, much better breeds for egg laying along with Dual Purpose, old breeds that are so much better at both egg laying and taste, that I leave the "Franken Chicken" to Tyson Foods, they are the folks that had it developed.
If you need to fill your freezer fast, then the Cornish X is a decent choice but otherwise not so good.