The 9890 post talks about events that “don’t have reminders and you can’t control the source”. All events on the upstream calendar I’m syncing originally have notification settings. So the problem doesn’t seem quite similar. Actually if I could get the original calendar notifications, that’s nice, because I have to set them in its web interface, to get reminders in the computer.
 The ICS calendar I sync comes from ms-office-365 (it can publish to ICS or HTML), sorry that I forgot to mention that on the additional information I included (not sure if office-365 has known issues). As it’s the calendar for the office I’m not sure if I can share it (will ask), though it’s my personal calendar there. I thought that was a common misbehavior though.
I’ve tested with curl and I got:
So, at least 30 seconds, I guess?
As said, rather make it configurable. This is a real-world system…