You can begin to harvest potatoes any time after the plants have flowered. Just carefully reach down into the soil of your container and pull out some potatoes.
You can also wait until the plants turn yellow and die back and then harvest all of the potatoes. The easiest way to do this is to turn the container over dump it out into a wheelbarrow or onto a tarp. Then you can freely paw through the soil to find all of the potatoes. You may find some really tiny potatoes - don't chuck them- those can be some of the best and sweetest.
Cook your potatoes or store them and enjoy!