The need to replace your blade will depend on a variety of factors.
As a guide, it is advised to visually inspect the condition of your lawn mower blades in between mowing sessions.
If you notice chips in the blade, rust or a blunt cutting edge, consider replacing the blade.
Another tip is to look at the cut quality of the lawn itself - if you notice that the grass is damaged (torn grass tips rather than clean cuts) after you’ve mowed your lawn, this may be an indication that your blade needs to be replaced.