Buy the 2015 Old Farmer's Almanac!
What with weather conditionsWhat with weather conditions (not just now but since you planted) and the potential variability of any given area, we can't say exactly. Watch for the tops to become yellow and begin to fall over. Stomp (per our advice on this page) and pull the onions when the tops are brown. Here in New Hampshire, that's usually in late summer.

2015 Garden Calendar2015 Weather Watcher's Calendar2015 Recipes Calendar2015 Engagement Calendar 2015 Everyday Calendar2015 Country CalendarNew Year Cross StitchLobster Rope Doormats