Have you tried playing some video and pressing * then find that sleep timer setting and change it to off?
You can use the RPi to code on Roku with. The person above isn't following 100%. The RokuTV is the Roku and TV together. So it has some extra "annoying" features like sleep timer.
I have a RokuTV and this was the very first annoying thing to get disabled. Second was Volume mode. Turn this off or it will decimate everything to stereo PCM. Third was choose Dolby+ for audio and not use AUTO or anything with DTS since pass-through for DTS is broken presently on Roku firmwares.