Must a woman who has become unable to fulfill a vow to fast six days of Shawwal after 25 years of commitment offer an expiation for an oath every year or only once?

A vow of obedience must be fulfilled. The ruling concerning a vowed fast is the same as that of an obligatory fast in Sharia. So, whoever is permanently unable to perform it due to old age or chronic illness, then it transitions to expiation (fidyah), whereby they feed one poor person for each day.