Spinning 10 unique pokestops in ~30 minutes or less (some say ~45 minutes or less; I try to be on the safe side), with no more than 9.9 minutes between spins, makes the 10th pay out with double XP and 6-9 items (reportedly, up to 10 if an egg is present; I've never noted this). It's okay to hit the same pokestop multiple times within the duration, but that pokestop counts only once toward the 10.
Apparently, there's an extra bonus if you can manage 20 unique stops, but it resets after that.
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Spinning 10 unique pokestops in ~30 minutes or less (some say ~45 minutes or less; I try to be on the safe side), with no more than 9.9 minutes between spins, makes the 10th pay out with double XP and 6-9 items (reportedly, up to 10 if an egg is present; I've never noted this). It's okay to hit the same pokestop multiple times within the duration, but that pokestop counts only once toward the 10.
Apparently, there's an extra bonus if you can manage 20 unique stops, but it resets after that.