As with any living thing, the amount of time will differ depending on the type of grass, germination rate, weather conditions, and so on. A sodded lawn can be mowed much sooner than a seeded lawn, even as soon as two to three weeks after planting. (Most say to wait at least three weeks.) Grass doesn’t grow by calendar dates, though, so before you get out the mower, test for root development: --Tug test: Pull up on several sections of the sod after two weeks. If the sod pulls up easily, you don’t have good root development yet. If the sod feels rooted to the ground, root development is coming along. Let the new sod grow one-third higher than the mowing height before you mow. In other words, once the roots are established, you can treat it as you would an established lawn. If you prefer to mow on the lower side, let the grass grow to the high side of its range before the first mow. Then bring it down gradually. A seeded lawn will take much longer before your first mow, up to two months. (Sod, sprigs, and plugs won’t take quite as long — three to six weeks.) Treat it as you would an established lawn: Mow when it reaches one-third higher than the recommended mowing height.  Follow the other lawn tips like not mowing when it’s wet and using a sharp mower blade. This will ensure you don’t pull the new grass roots from the soil but give them a nice clean cut.