As Hawkwalk has already stated.. it depends on which type they are.
Wait until they flower, and when the leaves/stems wilt back give it a week and they should be ready.
We have already dug up all the first earlies at 12 weeks.
We started to lift the second earlies yesterday at 14 weeks.
The main crop have already flowered and are now dropping back at 14 weeks. So it looks as though they will be ready soon, but they can stay in the ground until they are needed.
UPDATE: No flowers had appeared around 10 weeks after planting the chitted potato so I decided to have a dig and see what was there if anything. I dug up 15 potatoes! Was very impressed!!