We recommend using DriveHQ FileManager client software's folder synchronization feature. You can create a cloud-to-local sync task to sync a cloud folder to a local folder. DriveHQ FileManager is more efficient in monitoring the cloud folder for changes, esp. if the folder has a lot of files and subfolders.
Using FTP's LIST command probably is ok, though you need to be careful not to create too much load (relative to your plan level). If you need to list a folder every 15 seconds continuously, using FTP has one disadvantage in that it either requires a persistent connection, or it needs to log in every 15 seconds.
If the folder contains a lot of files and subfolders, the LIST command is not efficient. How frequently is a new file added to the folder? If the frequency is much lower than once per minute, then you can check the parent folder first. For example, if you want to monitor the folder "/Folder1/Folder2" for new files, instead of "LIST /FOLDER1/FOLDER2", you can "LIST /FOLDER1" first. Assume FOLDER1 only contains FOLDER2, then it is very efficient to find FOLDER2's last-modify time. If FOLDER2's last-modify time has not changed, then you don't need to LIST the contents of FOLDER2 as it has not changed.