How long is too long to leave a sprinkler on depends on sprinkler type, soil, and slope, but generally leaving sprinkler on more than 30 minutes for fixed spray heads or more than 60 minutes for rotor heads in one session is too long and causes runoff, water waste, and plant problems. Too long causes puddling, runoff onto sidewalks, soil erosion, waterlogging, root rot, and fungal diseases from saturated soil, plus high water bills. For spray heads, more than 15 to 20 minutes at once on clay soil causes runoff, so cycle soak method of 2 shorter cycles is better. For rotor heads, more than 60 minutes at once may cause runoff on slopes. Signs too long is water running off, soil soggy, mushrooms growing, and grass yellowing from overwatering. Most lawns need 1 to 1.5 inches per week total, so leaving sprinkler on for hours delivers excessive water. As rule, never leave sprinkler on more than 30 minutes for spray and 60 minutes for rotor per session without checking soil absorption, and use timer to avoid forgetting and wasting water.
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