Suppose we have an elderly person who erupts and becomes a nova.
They are, let's say, 90 when they erupt. We'll assume that if they'd stayed a baseline they would have lived to 100.
Those are our assumptions.

Now they're a Nova, and they've taken enough dots in Slow Aging that their life expectancy is doubled.

Do they have 20 years to live (they were 90, they were going to live to 100, that's ten years, double it)

OR

Do they have 110 years (they were going to live to 100, double life span they live to 200)

Though...