t doesn’t matter much when you back up unless you have technical or personal constraints. Technically, it may be necessary to execute backups only during suitable windows of downtime. This might be at night when you don’t need your computer or if the amount of data is large enough to have the backup process take hours. For consumers, it makes sense to trigger backups at least once a week if you modify files a lot. This you could do before shutting down your PC in the evening.