The period between painting a radiator and switching them back on will probably depend on the type of paint used, the natural temperature and humidity of the room and on the number of coats applied. Typically speaking, paint manufacturer's usually say 24 to 48 hours after the final coat but always check the drying instructions on the back of the tin before switching radiators back on.
Switching radiators on before the paint has dried may result in the finish remaining tacky for longer. In the worst cases it may ruin the final finish.