Lawn Mower Blades should be sharpened every season or every 2 to 3 years or 20 to 25 hours of mowing lawns.
Lawn mower blades that are getting dull and need sharpened will cause your grass to cut unevenly or tear your grass so if you notice a bad cutting job on your grass then you likely need to sharpen your lawn mower blades.
A good sharp lawn mower blade will cut the grass properly and give a nice smooth clean cut of the grass so if you notice any irregular spots in the grass you're cutting then the lawn mower blade is probably dull.
Other signs that your lawn mower blade is dull and needs sharpened is that your grass blades are torn at the ends.
Or you have missed patches or uneven spots, a dull lawn mower blade is likely the cause and you can sharpen your blade using an angle grinder or a bench grinder to keep it sharp.
You can also pay $5.00 or $10.00 to have a lawn mower shop sharpen your lawn mower blade for you to get it really sharp.