It depends on ambient temperature, on a 80 degree day with AC running on max. cooling and hot engine the fan should cycle on and off. If the temperature gets very high the fan(s) will stay on constantly. If the fan does not come on it could be due to faulty switch, bad relay, and burnt fuse of bad fan. You have to go through a systematic trouble shooting and if you are not versed then its better to take the car to a good technician