I don't suppose anyone can give you figures, but it should be far higher in header files than in source.
I'd expect the header file for a class to document all the classes/functions/etc publicly accessable by including that header file. That can be quite a lot of lines to document a function whose prototype is a single line (no, just choosing good names for functions and their parameters isn't enough - it can be, but often isn't). Three lines of comment for every line of code would not be excessive.
For actual code, it would be much lower. I don't agree with the extremists who think you should aim for zero comments, but certainly if you think you need comments you should first consider whether the code could be made clearer.
