Q: how long is the grace period if I ran out of iCloud storage before my photos & videos are deleted?

iCloud will not remove your files itself if you ran out of storage. It will just stop upload new files to update your existing backups. You can remove files by yourself if needed. Here is also the article where you can find more information how to manage your iCloud storage: Manage your iCloud storage - Apple Support

It is not clearly defined.

http://www.apple.com/legal/internet-services/icloud/en/terms.html