2013 Toro MX5060, Kawasaki 23hp.  Sometimes it gets real dry and the mower kicks up a lot of dust, especially under all the big trees I have.  I take off the paper filter and tap it on a tree trunk getting a lot of dust out of it.   I have a foam pre-filter on it but lots of dust still is on the paper. Question is how does one know when to toss it and get a new one?    Or can you keep using it like this?