If have just sharpened lawn mower blades..how quickly will dull them if run over ant or mole hills in lawn?

I sharpen or change blades every second cut. It does a better job. I keep four sets of blades. When one wears down too much I discard it. To tell if they need to be sharpened I simply feel the cutting edge. This will tell you if it's dull. Use common sense. When you sharpen your blade, balance your blade centered to make sure you don't sharpen on one side more then the other. I still have my 10 year blade. I rotate it with the newer blades. I buy a new blade every two years. This is not too bad. I would not sharpen the blade every week, just every two or three cuts.