I’m feeling pretty bummed that my session for tomorrow was canceled, but also a little sad that he waited until I reached out to confirm our plan. How late do you think is too late to cancel the session?
I'd say where circumstances are in your control, 24 hrs +. For me it's not just about leaving some notice before the session, it'd be so I don't deprive them of potentially having that slot for someone else. Talking about pros here of course which applies in your case.
In general, I have a 48-hour policy because that is the minimum time I need to find another client to fill the time slot. 24 hours is cutting it close, especially if I am fully booked the day before the cancellation I may not have time to do outreach to find another client on such short notice. I require a 50% non-refundable deposit from first-time clients since they are the most likely to cancel, and I haven't established trust with them. I am more likely to have some grace with regular clients, especially if it is something out of their control, like getting sick, I know them well enough that I actually believe them when they say they are sick vs a brand new client may easily be lying to get a refund. I also utilize booking software that sends out reminders three days before, one day before, and one hour before the session so they can't say "I forgot" or "I didn't put it in my calendar". One time I had a client who booked me as a back up plan. His plan A was to see a friend, and plan B was to see me, so he planned to cancel with me if the friend was available. Some people have no respect for our livelihoods which is why deposits and firm boundaries are essential.
If it’s canceled lower than 12 hours away, you can ask for half per site suggestion because of loss of income.