I interpreted it as a period of time for each of the splices. The first one (the necromancer with the familicide spell) lasted just a few minutes, so that archfiend only gets her for that much time. The other two splices both lasted until she lost concentration while fighting Xykon. So each of those fiends gets 20ish minutes with her. My understanding is that just one of them is using his time right now. I suppose they could then burn another fiends time, but why? As you say, they got what they wanted with this.
is the strip in question. Seems clear that just one of them is using his time, and his time is 20 minutes, 35 seconds. Um... Which while interesting and all that still leaves us with the definite probability that V will come back "soon". Whether that means that she saves them, or some other random thing will happen is pretty much up in the air. Edited, Jul 26th 2013 3:10pm by gbaji