When you've filled 90% or more of your Network Folder, you'll start receiving an email every week telling you that you've almost reached your quota. You'll also receive a notification email if your allocated storage amount is reduced because of an enrollment status change, or if a temporary increase expires. These notifications are a useful reminder to clean out old files and free up additional space.
If you move a large amount of data to your Network Folder in one go, the file transfer will fail if you don't have enough space. Once you've reached your quota, you won't be able to store any more files in your Network Folder until you free up space by deleting files or increase your quota through the Account Center.