I would love a feature that limits time since I have the TV in my daughter's room and have caught her trying to watch it at night. If we have it for cell phones I don't see why it is not available for Roku. Seems like it would not be too much to ask that someone put it in a s a feature. In fact it would be something that would make Roku stand out from the others.
Might be smarter to limit at the router level - this gives you control over ALL devices on your network. Several routers now have a feature which can set timers and schedules for devices/groups of devices. For example, we have an Amplifi HD router which does exactly this: https://amplifi.com/amplifi-hd
I did it during Thanksgiving this year with a trusty inexpensive basic Timer from Philips. I set it to go on and off whenever I want it to BEFORE my guests arrived. And I positioned (plugged it in) at the electrical wall socket rather than hanging it up on a power strip for all to see (and possibly defeating my purpose of gentle crowd control).
Sometimes simple solutions are the best way as even adults don't look too far when their attention is diverted/distracted elsewhere.