If you have a scale (or can borrow one) you might want to weight them now to see how their doing. That would answer the question as to when to slaughter them.
FYI, I just checked the weight of the roasters at the local grocery store. All of them were less than 4lbs dressed (I've never really paid attention before). If using a store-bought roaster, our family needs two birds for one meal. At exactly 7 weeks old, ALL of my straight run birds are over (or at) 5lbs live weight, some nearing 7lbs.
So, short answer: I'll butcher most at 8 weeks and save some for another week or so. All of mine should be bigger than store bought birds, with some being considerably larger.
Keep in mind that the longer they are alive the higher your costs per lb and the lower your feed conversion rates. The CornishX is often mentioned as having a 2:1 feed-to-meat conversion ratio, but that is only true when butchered as early as possible.
By way of comparison of price per lb, most chicken in the store was going for 98cents to $1.29 per lb, and for pricey cuts (boneless breast) it was as high as $5.19/lb (at a local small town grocery, which would be higher anyway).
