Only as needed. All of my Phals are in Orchiata bark (Power#5 size), and I foresee it lasting a long time.
LECA: light expanded clay aggregate. They are those small-ish, orange-colored pellets. Leca is an actual brand name, Hydroton is another. I'm sure there are many more. As rbarata indicated, some of us will mix it with bark or other media. It's also commonly used in semi-hydro growing.