You can calculate how long to water if you know your gallons per minute (GPM), Radius and Arc of your sprinkler nozzle. I created an Excel spreadsheet that will calculate the number of minutes needed to water to a given depth for a single sprinkler head. Type in your nozzle parameters for GPM, Radius, Arc, and specify a watering depth and it will tell you how many minutes to water to reach that water depth.
Sprinkler heads have head to head coverage so specify half the amount of water for the area you want to cover.
If you know the water pressure at the sprinkler head you can get the GPM of the nozzle by looking at the manufacturers performance chart. You should already know the radius and arc of the sprinkler head.
Email me if you want a copy of the spreadsheet.
ItÂ’s also listed on Google, but you need to sign up for a free account.
http://spreadsheets.google.com/pub?key=pGtsTbe3YK-h91OKgHRSzCw&output=html&gid=0&single=true