Check this page; you'll find an example of "extracting" secs/mins/hours... that elapsed between two timestamps. It doesn't look very elegant, but perhaps you'll be able to use it as an idea for your problem.
For some reason Oracle haven't quite got around to overloading SUM and AVG for interval types yet. (Presumably they are too busy working on 10g for Intel Mac.) In the meantime you have to roll your own.