On display at a grocery store with fluorescent lights on all the time, not very long… can be about a week. Here are some blogs posted earlier:

• Greening of Idaho Potatoes
• What's with all the Green on my Potatoes

University of Idaho Extension: Options for Storing Potatoes at Home

Also, the University of Idaho conducted a study inhouse and they received the following results:

It will depend upon the intensity of the light and duration of exposure. It also depends upon what you consider green- any slight hue or green green. Regardless, in our studies we saw green develop between 2 and 4 days, but this was under 23 hour light exposure.