As a coach, my official policy is 24 hours notice, but I'm flexible. I have kids, so I understand that they come home from school feeling ill unexpectedly. Plus, things happen, like dead car batteries and flat tires. If someone makes an effort to let me know in advance, it's no big deal.
When last-minute cancellations and no-shows without warning are a regular occurrence, that's a bigger issue in my mind. After the second time, I set up an automated "invitation" and unless the parent acknowledges that they'll be there, I don't expect to see the skater.
When a skater asks for a particular lesson slot, then cancels at the last minute every other week (or more) for non-emergencies, it's frustrating to the coach and the skater. The skater doesn't progress without regular lessons and practice, and the coach loses income. I have one skating mom who is great and always tells me, "If so-and-so cancels, call me and DD will take a lesson." That doesn't work if I'm cooling my heels at the rink and the skater just doesn't show up.
Earlier this season, one of my most reliable and consistent parents just didn't show up with his two DDs. He's always punctual and really great about cancelling ASAP. That day, the weather was bad and I had seen several accidents on the way to the rink. Of course, my imagination started to get the better of me, but fortunately, he sent me a message saying it was a dead car battery. Still, you worry about unexpected absences.
Cell phones, texts, emails and front-desk messages have reduced the number of no shows, fortunately.
If the skater's injured, I make it clear to anyone who fills the slot that's it's a temporary arrangement.
In the past, most of the injured skaters chose a different slot because of schedule changes, so it was a moot point.
Some skaters take "breaks" from skating for vacations, trips, special projects and even school. (Sept's always quiet.) I explain to the parents that I really cannot hold a time slot open indefinitely. They understand and schedule based on what's open when they return. Often, a different slot is more convenient for them, anyway.