Arrives with X time runes. Remove one each upkeep. When you remove the last, sacrifice this.
If you remove the last time rune for some other reason than the fading ability, such as from Time Spiral, Seer, or Tinkerer, you still must sacrifice the fading thing. — Sirlin, 03/11/16
If you somehow have something with fading in play with 0 time runes (probably because you made a copy of something if fading), it won't die from fading anymore. — sharpo, 03/14/16
X is not a limit to the number of time runes you can have on that card. For example, you can play Shimmer Ray and discard 4 cards so that it has 6 time runes; you're not limited to 2 total. — Sirlin, 03/19/16