Too little moisture will cause grass seed to become brittle and dry but soaking grass seed will cause rot. The goal is to adequately water the seed to keep it moist -- not soaking wet. Do so the entire germination period, which ranges from five to 30 days.
There isn't a single watering amount for grass seed because there are many factors that can affect germination. Seeds may be at different depths, the weather fluctuates, and different types of grass sprout at different times.
It's best to water grass seed in the early morning hours. If that's not possible, aim for the late morning. Watering in the evening increases the chance of disease.