How long does it take grass clippings to turn into compost?

Grass clippings left on your lawn after mowing will decompose in 3–4 weeks on average. Within 1–2 weeks the grass clippings will often no longer be visible, because they will reach the soil level and begin to break down. Grass clippings added to compost will break down fully in 1–3 months.

Are grass cuttings good for soil?

In addition to doing all the things other biodegradable mulches do (retain soil moisture, block light to weeds, improve soil), grass clipping mulch contains so much nitrogen and potassium that it serves as a supplemental fertiliser. ... It may also deter some pests.