2 to 3 inches before each mow.
The secret to a care-free lawn that requires less water, fertilizer, and mowing is all at the roots. Work on improving the soil by aerating and using organic-base fertilizers, and your grass roots will dig deep to find their own water and nutrients.
Get a mulching mower. Leaving the grass clippings on the lawn not only saves you the time of gathering and dumping bags of cut grass, but it also returns nitrogen to the soil so you won't have to fertilize as often.
Put in a ground-level edging surface. A landscape timber or curb at the same level as the lawn means you can set one wheel of the mower on the curb and edge the lawn at the same time that you're mowing.
Make a high cut. The crew cut is outdated. The type of grass you grow determines how high you can set the mower, but most lawns can be allowed to grow 2-3 inches tall before removing the top one-third of the blades. Longer grass shades the roots and has more leaf surface for making food, so you will water, feed, and mow less often.