I have always clean my k&n by looking at the color if I know it has some time on it and starts to look faded thats when I clean it and if your worried about over oiling just dont spray till the oil is dripping out and give it plenty of time to dry if your worried its over oiled I think its actually easier to just have two filters one already cleaned in a good bag to keep stuff off of it so you can just swap over real quick and your not hurried into having to force the filter to dry
I always cleaned mine on the same schedule as a paper replacement (about 30k miles). They always seemed to be getting a little grimy by then.
As far as the oiling it doesn't take much oil. I run a very thin bead down each raised edge of the element and then allow it to sit for 30 minutes for the oil to wick out. Then using the sun or a good light I look through the filter for dry spots and spot oil as needed.
Friend of mine let one go too long and it started coming apart. Took out his mass air sensor.