I don’t know if you have a Mac or PC so you will have to pick the forum.
You should also mention what software you are using to backup your files.
Normally you backup all your files the first time and then only those that are new or updated the rest of the time, however it all depends on what you want and the backup software.
If you are using smartware you can see the manual here.
Mention also, that there are different kinds of backups. If the Windows backup is used, it makes an image file of all the files on the PC HD, so it could be totally restored in case of failure. There are also backup programs that only back up your data, and/or what folders you specify.