Again, if you view it as a buff rather than a heal, it makes perfect sense that the buff lasts longer the higher your Cha. Compare Toughness, which refreshes only once per rest and is only 1 HP per level. With DV you have a feat that gives you twice the HP of Toughness, plus increased movement speed. Its duration is shorter, but with high enough Cha, you can keep it up virtually all the time.
If it lasted three rounds and you did not get hit, the hp would be lost, and you'd have to renew it. To me that's inferior to having them around until you get hit. If you are losing them through being hit every three rounds, that seems a movement back to the time when it was considered OP'd because of truly being able to spam it.