I think that when Belkar dies -- and I assume he will; the oracle hates Belkar but was straight with Roy to the point of trying repeatedly to warn him of Xykon despite Roy's too-clever question structure -- it will be the end of Belkar. No "Oh, but now he's an undead/golem/construct/polymorphed talking lamp" nonsense. Durkon being made a vampire was a plot point Burlew felt was very important to Durkon's character arc and I don't see him reusing it just as a "gotcha" for people expecting Belkar dead. I'd actually say that Belkar's death will be the culmination of his arc (I guess it'd sort of have to be) so there won't be any cute tricks to mitigate it.
But, yeah, I agree that it won't be until well into the last book since I can't see the next couple years going without 1/6th of the OotS (or 1/3rd depending on Durkon).