If you 1) don't plug it in at all for several weeks, including 2) ignoring low battery warnings for multiple days near the end, then you're looking at like 15min of making coffee and taking a walk around the building to let it charge enough to finish out the day, if, say, you sat down at your desk in the morning and it was already dead. Or 15min of using your trackpad if you're using it with a Macbook. But it's the kind of thing you'd have to be almost trying to do to have it happen even once a year. The crazy-good battery life and very fast charging are what make it such a non-issue.
If it had an edge plug I'd probably have started off using it plugged in all the time, though, just because I'm so used to battery powered peripherals being so shitty that not plugging them in when there's an opportunity is a bad move. Kinda like how it took a period of adjustment when I started using Macbooks to get used to my laptop being usable long enough on battery power that I didn't need to constantly be looking for power outlets, or carting my power supply everywhere I went.
I don't know if it would be a once-a-year event. That happens more frequently to me with the the Magic Keyboard 2. Thankfully I can connect it and keep working.
