Cool! I'm guessing you're using some sort of marker entity (armour stand?) to tp the player back to the death location. I'm guessing you know that Minecraft tries to unload all chunks every 45 seconds which will be your time limit at the moment, since you can't target entities in unloaded chunks.
Thinking about this has also got me wondering how the warps on the server work, since you should be able to apply the same principle here (Boss Man?).
For your needs you could permanently load the chunk (well not permanently but you know what I mean) but that doesn't seem viable for saving warp locations on a server because of how these could accumulate over time.
I vaguely remember seeing a video a while back where someone tp'd an entity back and forth across the world border to somehow detect the coordinates of a player and store it as a scoreboard objective or something. I'll see if I can find it, although there's probably better ways of doing it.
Oh and I know bugger all about command blocks so you probably ignore everything I just wrote.
P.S. I spotted a typo
@Tez1010 iff
:P