I use WFP now so it's probably not so relevant.
However, when I did trad, I would usually quote the first clean as glass and frames to bring them up to spec - though heavily ingrained staining would not come out. I would make it cleat that subsequent cleans were for just the glass and removing any liquid that I put on the frames (e.g. sills). If they wanted a frame clean periodically (once or twice a year) the charge would be the same again as for glass+sills). Some people had frames done once a year. Most didn't bother. If there was a bit of muck on the frames that bugged me, I would usually wipe it off anyway even if they weren't paying.