Dave Clarke told me to change oil every 3 hours (think it was 3, mightve been 4) and to change the paper filter every 3 or 4 oil changes. I believe you can get re-useable mesh ones which can be cleaned but the paper ones cant be cleaned i dont think.
As crispin said, clean the oil screen every time you change the oil. i also use solvent (brake and clutch cleaner) and then blow dry with air line (gently)
Keep yer oil clean and your engine should last for a long time !