I remember this having come up in some saga, and I don't think we reached a consensus, or at least not one single, obviously correct interpretation.
IIRC the case at hand was my rusticanus learning rope making. He hired a craftsman in game to train him for a season or two with an express promise never to make a competing business. No problem with the restriction, since he wanted to make ropes as charged items, which could be laid out in a closed circle, triggering a circle/ring effect when the circle is closed in a tied knot.
One of the spells he planned was a circle/ring variant of Demon's Eternal Oblivion. IIRC the troupe voted down the idea of a repeating effect, draining might every round for the duration, and I think he ended up with a momentary effect, draining might only, but from every demon within the circle.
The background urging him to learn this trick at the time was an anticipated attack at the wedding of a friendly noble. The neat trick about circle spells is that you do not have to sense the demon. You only have to sense the circle to affect every demon inside. Thus even the momentary spell is a huge benefit. The rusticani rope means that you can safely make a 300' rope and enclose the entire church. Or at least, that was how we reasoned. We may have pushed the limits for charged items and circle spells.
But I digress. The point I wanted to make was that I have not seen a compelling case that repeated drain to beings inside the circle is or is not allowed by the rules.
In the end, it never came up in play, because we jumped the gun and had to fight the demon lords in open battle instead.
What is clear, I think, is that such a sustained effect would apply every time the demon enters the circle, if they can enter without breaking the circle. So then you only have to lure the demon into and out of the circle.