I don't think this does what it looks like, but I'm not sure. I can't remember what it does do.

­Venarian Gold (with its current errata) shows the templating required for a version of the effect that actually works (or the other obvious way to make it work is by using counters, a la Quarantine Field or Endless Scream).

Gain X life until ~ leaves play?

Y'all remember this set is FOR designs that don't work, right? There are several ways you could do it (even just "lose that much life" might work? Although is still a question if you SHOULD do it cf illusions of grandeur).

But if you did print this, what value would the second x take? 0? Anything you like? Something else? Or just not possible?

Yeah, this sortof approaches working for the ETB trigger (Polukranos, World Eater shows they're happy to tweak the rules to make this kind of ETB thing work if they consider it worth it), but there's no way the LTB trigger could happen :)

For what it's worth, thinking from a strictly power level perspective, I think this card would be printable even if you cut the "lose X life" line. It doesn't seem that much more powerful than Stream of Life, when you think of it.

Yes, that variant would be totally reasonable. It's supposed to be unclear, not balanced :)