After applying fertilizer, you should wait at least 24 hours before watering your lawn. After this period, you want to water your lawn with 1-2 inches over the course of the week. How long this will take to achieve depends on the size of your lawn and the effectiveness of your sprinkler. “Run the sprinkler for at least an hour after fertilizing to ensure the fertilizer gets absorbed into the soil,” says Debra Lagattuta, a master gardener based in New Jersey.