When the 7th lord is in a dusthana, namely, 6, 8, 12 (provided he is a benefic) and when he has a bad navamsa, there is a high chance of spouse's death.
Naturally you will find the spouse's lagnadhipa weak, 8th afflicted etc. etc.
For a karkataka lagna, when sani is in 12th house along with Khantakaraka Ketu, and when the spouse's horoscope has got rahu in dhanu lagna,
guru in neecha, and 8th lord in 9th house, we can assume an early death of the spouse.
In this particular case i am analysing now, From Rishabha Chandra, as Kuja is in 12th, in neecha amsa, the chances are high.
So whens the period?
Guru dasa, Sani apahara
Guru dasa, Ketu apahara or Kuja apahara
Sani dasa, Guru/ketu/kuja apaharas