Half-elven Lifespan

[Fingolfin II]

how long did the Half-elven have to make the choice of their kindred

I don't know if you'll be able to find a definitive answer to that question. In the Silmarillion, Earendil, Elwing, Elrond and Elros are given a choice by the Valar at a certain point of time, but it is not said when the children of Elrond have to 'choose', though I think it may be safe to assume that they had to choose by the time Elrond departed over Sea (as evinced by Arwen saying that there 'is no ship' that could now take her across the Sea upon Aragorn's deathbed despite Legolas building a ship and taking Gimli with him, but anyway...). However that supposition is contradicted by the fact that Elladan and Elrohir decided to stay in Middle-Earth (at least for a while longer) and it is not made clear by Tolkien what their choice is and when they chose their paths, or if by staying in Middle-Earth they had 'chosen' a mortal life.

While that really doesn't answer the question, I suppose it is one of the ambiguities of Tolkien's writings- kind of like what does he mean when he says that Elves remaining in Middle-Earth 'faded out'; does that mean they became mortal and died, or went over Sea or what? I guess - like this topic - it's a moot point (unless someone points out evidence from one of the books I neglected).