DO NOT oil your apexi filter.....
its paper based and dry filter putting oil in it will probably screw it up......
the apexi you will just have to take it off and tap it on hard surface to get the dirt out.....
Apexi does sell a kit that replaces the filter media....
as for how long, just do it every service - wont take long....
with k&n ones though you will need to wash/dry/re oil....