Estimates time for a resume

This applies to almost any kind of skills, but I am using Project management as an example. I have run projects on and off across maybe 19 years of experience. How many years of PM work do I list on my resume? Also, I think that as you list more years, maybe an employer think you should have progressively deeper skills? I don;t do much costing, for example, and conducted projects only semi-formerly. How can I capture the essence of that for a resume?