I don't know the answer but the pensions turn taxable when the person receiving the benefits turns 65 so if I was going to make a bet, I would put my money on when the surviving spouse turns 65.  But for what it's worth, I'm not much of a gambler and I have never had much luck at winning when I buy a raffle ticket so take it for what it's worth.  @PATAX can now join me in a chorus of :

Gloom, despair, and agony on me
Deep, dark depression, excessive misery
If it weren’t for bad luck, I’d have no luck at all
Gloom, despair, and agony on me