If we go by its literal meaning, longevity of the marriage is the ability of the husband-Wife relationship to go for long in spite of the differences/problems/challenges and other reasons.

Now, longevity doesn’t always implies that marriage is going to be a happy one. It may be long, but no one can guarantee it is going to be happy also. (बाबूमुशोई, जिंदगी बड़ी होनी चाहिए…लंबी नहीं)

7th house is the house of legal partnership. Now, if we look from the society point of view, your spouse is also one of your legal partner. This house shows that what will be your response towards your legal partner whose profit and loss will be vis-à-vis your profit and loss. 8th House represents longevity of the marriage. In planetary terms, Saturn represents longevity and maturity. Venus represents love. Jupiter represents expansion and wisdom.

Lets come to your question best placement for longevity of marriage?

So, longevity of marriage will be there when placement/aspect of Saturn is in the 8th house.

However, if we have to see longevity and happiness at the same time in marriage, then placement/aspect of Venus and Jupiter to be in 7th house or with the lord of the 7th house. Additionally, placement/aspect of Saturn should be in the 8th house.