I think there are two potential problems:
- If the backup job is too big so that it takes longer than 15 minutes, you won’t be saving a new version every 15 minutes
- If you have other backup jobs, you will not get any versions while they are running.
Depending on why exactly you want resl time backup, I think the better solution would probably be to to put those frequently changed files in a Dropbox folder and let Dropbox do the real time backups (which it saves for 30 days). I guess in most use cases Dropbox will do as your short term backup and let duplicati take care of the long term backup.