User blog comment:Zelron/Ageless bloodlust (How long until you're hungry?)/@comment-2040576-20121014080048

Interesting point! I have not been able to play Dawnguard yet, so I am not entirely sure of the answer, but reading on the page about Valerica I found something that may provide an answer.

"As the passage of time has little meaning in the Soul Cairn, the waiting game continued for thousands of years."

Perhaps, time here does not pass normally as it does in the mortal world and so she did not suffer for so long without blood? That is the only explanation I can think of.