@darkus I have the RPI system 24/7, but I don't have this problem. The TV turns off the screen after about 1h (2h maybe). I have the screensaver/power options active in EmulationStation, if that makes a difference.
One thing to try is to move the HDMI PI cable to a different TV port - if you have more than 1. Mine is set HDMI2, which is labeled PC, and has the Game mode active.