Not exactly, it depends on the target hp at least, this post explains it.
What I would do is leave it as it is, but increase that cap to 2x
So the demon would start out with the 10 minutes, however killing things could build up a lifespan of up to 20 minutes, which I think would already be quite a significant change as it doubles your potential downtime between targets…
Sometimes small changes have quite a huge effect and it’s generally better to do it in small steps, especially with Funcom’s track record of going from one extreme to the other. If further modification is needed, it can always be requested again