At 7 weeks, your CX should already be a decent eating size. I usually process mine anywhere between 8 and 13 weeks depending on the weather, what I've been feeding them, and how big I want them. A couple of things to bear in mind.
First, I don't know where you live, but summer heat can be very hard on CX, especially as they get larger. Waiting too long, could result in some losses, depending on what your climate and ability to keep them cool is. If they look like they are struggling to walk and panting, those are signs that it may be their time.
Also, it sounds like you are processing these yourself and that are new to it. You may find it easier, physically, to process them before they get truly huge. Getting a 7 or 8 lb bird squared away on a stump, or in a killing cone, is going to be easier then wrestling with a 12 to 15 pounder.
One thing to think about is doing a test run now, with one of your larger birds and see if you are happy with the carcass size.