For how long an uploaded file stays available if it is not stored permanently?

What's the logic for how long an uploaded image to ucarecdn.com will exist? Some images I uploaded over the past day or so have disappeared.

By the default, every project has "auto file storing" option turned on. If you turn it off you'd need to store files manually. If you don't do that your files will be available for 24 hours after upload then they'll be deleted. Stored files are available until you delete them.