It may seem counterintuitive, but oftentimes, asking more of your athletes will make it easier for them to commit and enjoy training. There has been a great deal of research regarding religious psychology on the topic that applies here. If a religious leader asks the bare minimum of their followers - show up to church on Sundays, don't murder anybody, say a prayer now and then - that religion will die. But when they ask their followers to make incredible sacrifices - up to and including their own lives - that religion will spread like wildfire. It's part of why martyrdom is so effective; it's easy to believe in the promised rewards when you see what you have to give up to get them.
That's not to say you should ask your athletes to sacrifice their lives for a high school sports team. There is certainly a danger in letting your leadership take on a religious aspect. But asking too little of your athletes will often result in dissatisfaction, apathy, and a lack of belief in you and your program. Success is more rewarding when you work hard; the harder you work, the more rewarding it feels (even if the result is the same or similar). It also helps that hard work, in this sport, does result in improved results, as well as building camaraderie within a team (conquering the challenge together).
I would say the rule of thumb should be to never ask more of your athletes than you yourself are willing to give. You should always be the yardstick for hard work. That's not to say you need to run 100 mpw to ask an athlete to do so, but if you're demanding your athletes need to show up 6 days a week for a 2-3 hour practice and meets on weekends, and give it their best effort every time, then you had better be doing the same. Being fully invested, cheering for every race, constantly giving your athletes advice and encouragement, showing up early and staying late, all of it. And sometimes it can be as simple as just doing the run or workout yourself, when it makes sense and doesn't detract from your coaching in other ways, to show your commitment and belief.
Do all of that, and I think it is a very fair expectation to ask a lot of your athletes. At that point it becomes less a question of what is a fair expectation, and what is optimal for a given athlete. What that actually looks like will vary from 20 mpw with two light workouts all the way up to 100+ with double thresholds and racing every week, but as long as it is consistent with the amount of work you are putting into your coach, it is a fair expectation.