This is a very objective question really. It all depends on your ambient room temperatures. If you're room temp is like 20 C and your case is 25 C, then your CPU temps shouldn't go much over 40 C. But if you're in a hot place like me, and youre case temps can reach 35, hitting 50 C isn't that bad. So my advice is to try and keep your CPU temp no more than 15 C above the case temp at full load.
That oughta void your warrenty!