Mother, 66, had gangrene/ partial foot amputation, is not diabetic, had 3 stents in her legs due to poor circulation & heavy smoker. life expectancy?

Statistically...: Unfortunately, these effects you describe don't just have an effect on the feet and legs. The effect vital organs are effected as well. 5 years has been reported as an expected life span after an amputation, but this is just an average. It could be more or less depending on many other lifestyle factors.

Peripheral diseases: Unfortunately, peripheral arterial disease is as common as coronary arterial disease, we just never hear about it. Pad kills more people per year than breast and colon cancers combined. The rate of mortality after amputation (below-knee) is approximately 50% after 5 years and most people (60-70%) will lose their other leg within 5 years once the first amputation has been performed.

Many variables: It depends on the extent of the amputation, the success of the stent procedure, and the patients other co-morbidities.