It comes down in my experience how much sand is in your soil that will blow around when cutting, your effectively making a sand blasting cabinet under your deck , I have one customer who's going through 2-3 pairs of blades a year and others where blades last 2-3 years, it is wise to check your blades regularly (Like checking your oil) as blades wear they will become very thin and little bits will break off and come flying out at a zillion mph and if they hit something there is going to be damage
there is my 2 cents worth.