Cleaning the filter depends on the riding conditions. Lots of dust/water means more cleaning. Daily if necessary.
However, under normal riding conditions, usually once or twice per season. When the foam begins to develop discoloration and a bit of a sheen from dust accumulation, air flow will begin to get restrictive. Often,it will be more predominant on the side/end of the air box where the intake is. Sometimes, if the filter is really clogged, it will cause a choke effect and result in more burnt fuel and some power loss as well.
Bottom line is that if it looks dirty, it should be cleaned. If it looks a bit discoloured but not clogged, I'd run it a bit more before cleaning. If, getting to the air filter is more work than cleaning it, you might as well clean it while you've got access to it.
Yes. I've used it on chains too. Don't like to use too much on a street bike because it can be messy. On a dirt bike, it won't be as noticeable. Clean the chain with WD-40 and brush the bar oil on lightly.