You might want to let the pointy hair types know that the industry standard is 30 minutes of inactivity ends a browser session. This standard is not written in stone, but is generally accepted. The 5 minute granularity does not tell you much, but I had this same debate with the unwashed. Fortunately I won that argument.
I would also recommend this document NetGenesis White Paper that describes many different types of metrics that you should find interesting.
Richard
There are three types of people in this world, those that can count and those that cannot. Anon