Nasa likely said something along the lines of saying he'd be with her until the day he dies. Tsukasa rebuts saying something like that is a remarkably short time. Nasa responds saying that even if he dies he'll stay with her forever. Tsukasa questions him saying something like eternity is a lot longer than you'd think, and Nasa confirms/repeats his statement that he'd stay with her forever, no matter how long that may be.
This is why Tsukasa was wiling to marry him despite not knowing much about him, having wandered the earth for 1000s of years already alone and looking for someone who was willing to join her, even if it meant drinking the elixir of immortality.