Elves: truly immortal?

[Legolas]

However according to the film of TTT, she has to live until the 'long years of her life are utterly spent'. How long would she live? I believe in the appendices it's about a year, is it not?
And Aragorn tells her to find a ship to take her to the West doesn't he? But if she's chosen mortality she can't (right?) so why does he say it?

What the film said meant simply that with choosing mortality, Arwen would have to live until her death (sounds obvious) - but implying that it was a final choice and there was a chance that she would live longer than Aragorn. Arwen is only one generation removed from Elros, who lived 400+ years, in comparison to Aragorn who is way on down the line.

Aragorn telling her to find a ship is just Aragorn wishing she could go back with her father after he is gone - I don't think he understood the choice (a *permanent* choice).