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Pay attention to your sanitation and they can sit there basically indefinitely. If you're getting antsy you can also attemper the yeast by adding a little cool wort, stirring, and repeating a few times.
I have honestly never considered this issue with dry yeast. I've never had a problem with dry yeast fermenting just fine, either.I suppose it sits for an hour or so after I rehydrate it. But I've never given it much thought at all.
Since no one else has said anything yet - You can always just sprinkle it right in the wort...