If your vehicle is showing engine takes a very long time to reach normal operating temperature, here are the most likely causes and the parts you'll need to fix each one.
The thermostat regulates coolant flow to the radiator. If it fails and sticks in the open position, coolant circulates constantly, preventing the engine from reaching its optimal operating temperature quickly.
A previous owner or mechanic may have removed the thermostat entirely to mask an overheating issue, or installed a thermostat with a temperature rating that is too low for the vehicle.
The engine may actually be warming up normally, but a failing temperature sensor is sending an inaccurately low reading to the dashboard gauge or the engine computer.
On vehicles with mechanical cooling fans, the fan clutch can seize internally. This causes the fan to spin at full engine speed constantly, over-cooling the engine bay and delaying warm-up.
A stuck relay or faulty fan switch can cause the electric radiator fans to run continuously from the moment the engine is started, drawing too much heat away from the radiator.