ttramelle, Uh, if you are talking about meat birds, they eat meat bird feed all the way through their lives. There's almost never a "switch over", unless you wanna go with medicated feed for your first bag which is a bad idea to ferment. I feed 20% start n grow their whole lives. It doesn't make much sense to "switch over" a bird at 8 weeks when you are going to butcher them at 8 weeks? Some people even butcher at 6...
Which the answer is, 8-12 weeks is the appropriate butchering age. If they're big enough for your tastes at that point, then they are ready to butcher.
Chikee, the rule of thumb is free feed for the first 2-3 weeks, then feed twice a day, only what they will eat in 10-20 min, however much that is. It will change as they grow. There's no "set amount" because you want them to eat fast and grow big. There's no upper limit and no lower limit, really, because it is about filling their stomachs twice a day with however much feed that is. In the morning, feed them some amount, and come back in about 15 min. If the whole place is picked clean, feed more next time. If there's a little left, that's perfect. If there's a LOT left, feed a little less. Sometimes if they pick it clean you have to watch them eat the next meal to know how much more to add.