Oiling foam air filters

[neil-935ql]

Hi all , how often should foam air filters be cleaned and re-oiled ? , I am guessing about 10k miles , or should it be more often on a caterham ? My piper cross filter is looking dry with no oil on it . Could there be possible engine damage running a dry filter ? My car is a 1.8k on RBTB . On inspection the throttle bodies don't look that dirty , cheers Neil