The height of your lawn will depend on what type of grass you have. Perhaps the simplest rule of thumb is how your lawn looks. Looking untidy? Then it's time for a trim. Typically, you should only be removing 30–40 percent of the grass blade each time you cut. If you cut any lower than that, you may be reducing your lawn's root growth, which could hurt its long-term health.
Buffalo grass should be cut at 40–50mm, kikuyu is best kept at 40–45mm, whereas couch grass can be cut to 25–30mm. In shaded areas or the cooler months, you can keep your grass a little longer. And if you're still not sure, kick your shoes off and take a walk on your lawn—it should feel nice underfoot without feeling like you're sinking into it.