How do you know when your lawnmower blade needs sharpening or to be changed?

Heck, I used to sharpen my blade every other time I used it. Takes about 2 minutes once you get the hang of it. Just tip it over (fuel tank side up), remove the 9/16" nut holding it onto the shaft, put it in a vice, run a 4.5" angle grinder over it, put it back on and you're done. If the gr looks like it was torn apart, rather than cut, it needs to be sharpened. If some of the gr/weeds are knocked over and laying flat, instead of being cut, it needs to be sharpened. If you want to see what a really sharp blade will do, go ahead and mow half your lawn then sharpen the blade then continue mowing. Oh, you should be able to find a Chinese angle grinder for less than 20 bucks, it should already have a 4.5" grinding wheel with it. Buy a cutting wheel too, they come in handy. I've got half a dozen angle grinders, each with a different attachment.