Depends on a lot factors: distance, terrain (uphill vs. downhill vs. flat), weather, my physical condition, cost and convenience of public transportation (since I don't own a car). Generally, up to about a mile and a half each way assuming flat terrain and good weather.

I did walk from the Apple Store on 59th and 5th, through Times Square down broadway to Union Square at night in NYC in the dead of winter once. That was a ~45 minute walk in NYC sludge.

If I'm starving or extremely tired, off to Seamlessweb I go.