All viewers are valuable, especially those that are long time viewers. Regardless of whether they have subscribed or not they contribute to viewer count and hours viewed. Those that spend large amounts of time on the stream are often the ones that spread the word, retweeting live announcements and telling their friends to watch the streamer(s) they love.
Having alerts for "hours watched" milestones will help to give loyal viewers recognition which in turn boosts engagement and encourages them to keep coming back (because they want to hit that next milestone). Hours viewed is already tracked in SL Chatbot for currency earning so I imagine it's not too far a stretch to implement this as well.
Give streamers the option to set the hours watched milestones we what to recognise, and allow us to customise the alert for them as we can for existing alerts. I would think a limit of 5 milestones would be sufficient to make sure they aren't overused and make hitting those milestones extra special for viewers, though others may have their own ideas on what the max number of milestones should be.