That's a tough call. There's a lot of factors involved. Are you talking about residential use as in a homeowner or are you talking commercial use. It also depends on what type of grass you're cutting because certain types are harder on blades than others. Check your grass when you cut, if the grass has torn edges and not a nice even cut then sharpen the blades.
I'm a residential user but I only sharpen them as needed when cuts aren't even or flat. Usually once every 5 years or so. My lawn is flat bermuda grass so not heavy use or hard on a mower.
Depends on the type of grass and how much sand you have. I have centipede with some sandy areas.
I'm mowing roughly 4 acres every other week and I sharpen the blades probably every other mowing. I go through 3 to 4 sets of blades each year.
I've seen where others have put 40 hours on a machine without having to sharpen the blades. Mine get sharpened about every 3 to 4 hours of use and usually end up needing replacement after 8 to 10 hours due to the sand eating them up.