Items with monthly limits, such as links and reports, are counted when they are created, so deleting them will not reduce your usage for that month. See our pricing page for all features and their monthly limits.
For example, if you import 10 links, each of them counts toward your link creation limit for that month. Deleting them the next day will not reduce that count.
Another example: reports are counted each time a report is generated. So if you run it manually, it will count each time. If you set it to run biweekly, it will generate two reports a month and count both towards your usage limits.