Pretty much any time, depending what you are going to do with them.. I bought 6 in mid-February, and butchered them 10 days ago at approximately 9 weeks of age. The largest one(a roo) dressed out at 7 1/2 pounds, and the smallest(a pullet) dressed out at 5 pounds, with the others falling in between.
If you want fryers, butcher a little earlier than if you want roaster/broilers. It is all about your preference. The dressed weight of mine was approximately 70% of live weight.