An average/slow walker would probably take 25-30mins.
4 miles an hour is a perfectly acceptable walking speed.
To the OP: It depends very much on the terrain, and what sort of walking it is. If you're walking 2 miles through town, and taking it easy, I wouldn't be surprised if it took 40 minutes. If you're walking up a steep mountain path, it might take a heck of a lot longer. If you're walking on the flat, and deliberately walking at some pace, it could take as little as 20-25 minutes, but any faster and you'd really be walking at some pace and most likely look a little silly.