The weight of the chicken dictates the roasting time. Most chickens weigh between 1-2 kg. A 1kg bird or smaller will need to be roasted for about 1 hr at 200C/180C fan/gas 6.
The poultry is done when a meat thermometer inserted into the thickest part of the thigh reads 170 degrees and the juices run clear. A guide for cooking would be c.17 minutes per pound for larger birds.