A spatchcocked chicken can be treated as if it were chunks of chicken on the bone, so the same time for roasting (eg) chicken thighs on the bone. I would say 30-40 minutes depending on temp and how you baste them.
I don't like dry chicken and like the breast meat so wouldn't roast one in an hour - DP, who likes the thighs, would happily do one in an hour at a higher heat. It's personal.