There is no fixed figure as to how hot it should be. It will vary, adequate ventilalation, number of fans and what stress you are putting on it.
If it is running around 50 Degrees C then fine, it is ok. Like wise if its 20 Degrees C... If its in the high 60,70,80's then I would watch the temperatre closely...
Whilst it can run OK at 60-80 Degrees C it will not like entering the 90's and it could melt. Modern BIOS's will shutdown the computer when it gets too hot. Depending upon your motherboard you usually have some sort of monitoring software that you can open and keep an eye on the temps and some even allow you to set alarms.
So if as long as you have good cooling you could run into high 80's but anymore then have a look at your cooling.