What is the ruling on someone who vowed to fast three days every time he looked at unlawful things for six years, but did not fulfill his vow? Is expiation due for every time he broke his vow, or is a single expiation sufficient?

A vow to abandon what is unlawful is a vow of contention (nadhir al-lajaj), and the person who makes it has the choice between expiation and fulfilling the vow.

Therefore, you must offer expiation for an oath (kaffarat yamin) for every time you looked at what is unlawful, or fast three days for each instance.

The expiation for an oath is feeding ten needy persons, or clothing them, or freeing a slave. If one cannot find these, then fasting three days. If you are unable to perform this, it remains a debt upon you until you are able to do so, and it may be waived due to inability.

As for difficulty in performing the expiation, if it is extraordinary and leads to destruction or loss of a limb, it is waived for you. However, if it is ordinary, it is not waived.