That depends on a number of factors. What is your ambient temperature? How much ventilation does your case have?
Normally in a 24C room a temp of around 50C at idle (or less) would be great. While gaming anything under 90C should be fine. Most cards in well ventilated cases and not overclocked stay below 70C these days, according to my observations.
There is no such thing as an average temp. The only way there could be an average temp is if everyone had the same temperature house, same case, same cooling and same hardware running at the same speed.