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Assuming a 5400 RPM drive uses 2 watts less than a 7200 RPM drive and the battery lasts 3 hours and has a capacity of 60 W-hr, battery life would be 10% less with the 7200RPM drive, or about 18 minutes shorter.

The actual difference will depend on the two drives you are comparing because drive power use varies a lot. Look at the Tom's Hardware charts since they test power use for various hard drive load conditions.
 
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