You have somehow gotten the power off time set to 30s. The default is 5 mins, but is user controllable.
The variable is "TOff". Type this exactly in the home screen: TOff:=0
That is out of range, so it resets the value to the default 5 min value. The screen dimming time is not controllable on older primes, but is on new ones. If you mean you'd like to stop the screen from dimming, that is a different things but is also now controllable as well.
Although I work for the HP calculator group as a head developer of the HP Prime, the views and opinions I post here are my own.