Longevity of a chart should not be even mentioned (predicted) till the 24th year is completed which is the janma-riṣṭa period Maharsi Parasara
Bala-riṣṭa » 12 Years | riṣṭa associated with Moon as significator
Yoga-riṣṭa » 20 Years | riṣṭa associated with Jupiter as significator
Janma-riṣṭa » 24 Years | riṣṭa associated with Sun as significator
Study the following lessons carefully so that you know how to look for riṣṭa in a chart.
Tri-pāpa and Catur-pāpa
Maraṇa Kāraka Sthāna
Other Combinations explained
Shadow of Sun etc.
Things you need to study outside of BPHS
Bear in mind that Neṣṭā Karma and Neṣṭā Graha require considerable understanding as these planets can surely give marriage and make life a wonderful experience or the converse i.e. living hell.