Your account has 25 GB of storage. You'll probably never run out of space. In fact, a heavy email user who sends and receives lots of file attachments and archives all messages might use up to 5% of this space a year (usually less), so it would take decades to use up all of the available space. Also note that Google Apps restricts the size of file attachments to 25 MB, so you don't have to worry about a few large files using up your storage space.