One of the performance rules of Dynamics NAV with the proprietary database (not SQL Server) is to check the size of the db: it should not overcome the 80% full or your performances will decrease (if so, a manual database expansion is needed).
What about the SQL Server option? When you setup a Dynamics database with SQL Server, the standard (and recommended) option to give are to have an Unrestricted Growth and 10% Automatic File Growth.
Often, when you're in a production environment, by checking the database size you can find that it's over the 90% full. Is it a problem?
With the SQL Server option of Dynamics NAV, having a database 90% full is not a performance problem. But one of the common question is this: if I have the Automatic File Growth option, when SQL Server will resize my database?
It's not so easy to determine when SQL Server decide to expand the db size automatically, but an important thing to remember (in order to optimize your database performances) is that you don't have to rely on Automatic File Growth!
When SQL Server triggers the expansion process, it queues all the incoming disk requests and it adds new free space (10% of the actual size). Having a large database that will expand too often is a terrible impact on performances, so the better thing to do when you see that you're reaching the 100% is to perform a manual expansion of the db (inside Dynamics NAV you can do it by going on the File -->Database -->Alter menu).
P.S. remember to perform the resize task during off-hours... 