Normal temperature when in standby is 31°C
When Using Whatsapp, Facebook, Instagram, 90Hz FPS, 38 - 40°C
When Gaming 41°C - 47°C
37 = Comfortable to hold
38 - 41°C = Started to feel uncomfortable when holding
42°C› = Can't hold anymore.
Putting on a casing, you mean?
Just increase 1-3°C on every data I just provided just now, you got the temperature for the phone while casing is on.
Pretty sure people now understands that (Higher Temperature = Shorter Lifespan)