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I guess that's fairly subjective and would depend on the distances to and between your subject and the background. But I can say that I've taken plenty of tight portrait shots with my 24-105 f4 at 105mm/f4 and I've been happy with the amount of background blur, and the 70-200 f4 is even better. The 70-200 f2.8 is an absolutely awesome lens though and if I had the money to spare I would buy it.
EDIT: also if you don't need the length but desire nice bg blur then what about a 50mm f1.8/1.4/1.2 (depending on budget)?
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