Prince Imrahil and his mortality

[Formendacil]

Originally Posted by The Mouth of Sauron

When Legolas first set eyes on Prince Imrahil he recognised that despite being a mortal man he "had elven blood in his veins".

At what stage was Elven immortality removed from Imrahil's line? Did it depend on the percentage of elven blood, so that when it was diluted to a certain level death ensued?

It's a bit of a flippant response, but I can argue from the Legendarium that Imrahil was the last generation this Elven blood was apparent to Elven eyes (the issue of mortality or lack thereof is dealt with by the posters above).

To wit: Imrahil's sister (the same generation as him) was Finduilas, who married Denethor, mother of Boromir and Faramir. As there is no remark regarding Boromir from any of the Elves in Rivendell or Lórien--or from Legolas himself, we must remark that the visibility of Elven blood in the line of Dol Amroth ended with Imrahil's generation.