This is a bit like asking how big will a SharePoint content database get. The volume of report data relates to the volume of content and activity in your SharePoint environment.
I know this is not exactly what you asked but I think you should make a note of your database size, speed up the report frequence and monitor it for a week or two.
As well as changing the frequency that the reports are generate (to make them more up to date) you can also control how much report history is kept, by reducing the number of previous report results that are kept (cached in the SQL database tables) you could reduce the size of the database growth.
I hope that helps,