Default How hot is too hot (gt-i9192)

My gt-i9192 can get a bit hot. Reaching even 45 Celsius when I am outside using for example the maps app (needs gps, data and wifi (it claims it makes GPS more accurate and it seems to do)). or a game or browsing the web. Specially if the environment temperature is high (I consider 25 or so "high" for environment temperature). I am not sure if 45 is too much or if it is just okay fine. It does get uncomfortable and the temperature guard app defaults at 40 for the "too hot" warning.

How hot is too hot?