The longevity will mean that the wraith would feed for only a short time before he is overcome with a sensation not unlike a human would feel after eating an entire stick of butter. It would be too rich, and the wraith would have to stop.
My bet, based on...well...nothing, is that she will feel tired and draggy for a while - say a month - afterwards, like someone who has just survived a dangerous and debilitating illness, and then be just fine.
I don't watch Sanctuary so I don't know the metaphysics involved in Helen's immortality. However, I'd argue if she's anything like a Highlander-style Immortal, she could be fed on without dying. She'd just keep renewing her life-force.
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So does she age after the feeding?
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But it will provide some interesting theoretical discussions. :)