If it's just boiling then for a 1.2kg bird I'd boil for around an hour. Bring it to the boil then simmer. I wouldn't suggest boiling for that period of time because it will just fall apart.
I prefer using a pressure cooker, do you have one?
If you're using a pressure cooker then it won't need as long and it will cook better. You can also get away with leaving it on lower and forgetting it for 6 hours like I seem to do every time lol.