K&N air filters/cleaning schedule?

[Martin_A._Secrest@NEB.VOA.GOV]

Iım running w/a K&N air cleaner on my single Z-S.  Itıs been on there for about 
6 months, some 4 thousand miles (Iım guessing).

I understand that K&Nıs can be cleaned w/Formula 409 or equivalent, and 
re-oiled with mineral oil (if you disdain the K&N accessories, as I do).  But 
whatıs the opinion as to how often this ought to be done?

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Martin-not-yet-presbyopic-Secrest
74 Spitfire 1500
FM18943U