Depends on application of thermal paste, ambient room temperature, ventilation, airflow, material of your case, your motherboard, heat output of your GPU, number of case fans, arrangement of case fans, power of case fans, voltage, etc. Give me all of those things, or just tell me what the current temperature is and I can help you from there. Idle temperatures vary greatly, but they don't actually matter, only your maximum temperatures do. That's why when overclocking, you have to run a stress test like Prime95 (and to check for stability), and a monitor program. There's not much point in watching idle temperatures, that's like asking how many miles per gallon you'll get on a car that isn't moving.