Question
We live in a suburb of Detroit. MI.  My question is how long and often do you water the lawn.  We have about an acre of sod.  

Answer
Water the lawn so that the soil is wet to a dept of 6-8" deep. This typically means watering the lawn for 1-2 hours (thorough soaking). If you water for 10-15 minutes, only the top layers of soil get wet. You need to water deeply.

The general rule is to water once per week for 1-2 hours rather than 15 minutes daily. It is best to water infrequently and deeply (within reason) rather than frequent and shallowly.

It is best if you can water deeply and then let the top soil layers dry out moderately. This will encourage the lawn to grow deep roots in search of water and will also allow the grass blades to dry out limiting chance of lawn disease and weed infestation. If you water shallowly and frequently, the moist soil at the surface will ensure a shallow rooted lawn which is more prone to lawn disease, weed pressure and the warmer soil layers on top can also be difficult for the lawn in hot summer months.

If you must manually water the lawn (using osciallating sprinklers attached to garden hoses which must be moved) then water one section deeply (1 hour) and move the sprinkler and water another section deeply. Do this until the entire lawn is covered. It may help to water various parts of the lawn on different days.