You can certainly wait a couple hours before you pitch the yeast. However, if the pack is not fully inflated by then there is not much consequence of just tossing it in.
As per Wyeast...
3. Does the package need to be fully swollen before pitching?
No, The package can be pitched before activating, or at anytime during the activation process. The activation process "jump starts" the culture's metabolism, minimizing the lag phase.
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"5. How long should it take for a package to swell?
If a package is within 2 months of the manufacture date, the package should show signs of swelling within 5 hours and typically much faster than that. When the yeast is stored for long periods of time, they slowly consume their energy reserves (glycogen). When the energy reserves get low, the yeast are slow to produce CO2 and therefore are slow to cause swelling in the package. Improper storage at warm temperatures also has the same effect as long storage times.
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