Temperature once the 120mm fan is install depends on many things. Among them are air flow, thermal resistance, ambient temperature and output power. Fans reduce amp temperature but there is always the issue of noise.
That’s why installing a sensor is smart. It makes the 120 mm fan kit only run when the temperature is that high that forced-air cooling is required. In turn, this minimizes noise output. The result of a well-designed system is reduced noise, and in most cases the fan won’t even be needed unless the components are being overused.