We are asked this question a lot and it is very difficult to answer. The reason is it really depends on how much data entry activity you have. An organisation that is constently updating their records, adding new records to the system or deleting records will generate many changes in the database. An organisation who data is relativly static will not generate as many new rows in the database.
Each field change will generate a row in the database. For example if you change the first name from "David" to "Dave" there will be one row change. However if you had a record or delete a record there will be one for for every field affected. If a record has many gifts, actions, relationships etc on it then there will be many rows generated because not only has that record many child records but there is one row per field added, updated or deleted.
Each row does not amount to a very large amount of data in itself. A row consists of a number ids, plain text descriptions of the type of field and record, the changed date and well as the before and after values (See this article for more details). There are some fields that are not stored for example, embedded Word documents for letter templates or actions are not stored. There is also an option to prevent the storage of notepad descriptions. This reason for this is that some organisations store rich text in these with images and this will cause issues with the database if the size is too large. (In fact we recommend that if you are storing rich text in notepads e.g. images then you do turn off the tracking of these changes).