Watch out! Once you drink Amazon Prime's kool-aid, you may find yourself renewing the service... forever. (Or, at least, for a third year.) (credit: Rachel Murray / Getty Images)

Amazon isn't likely to release granular data about its Prime subscribers anytime soon, which means a privately run survey has a little more weight than we might otherwise give it—especially with a conclusion as resounding as the one drawn by Consumer Intelligence Research Partners on Tuesday. The firm's latest report estimated an Amazon Prime retention rate that may only be rivaled by alcohol, cigarettes, and other drugs: 96 percent.

That's how many Prime members elect to renew the service after two full years of use, according to CIRP's estimate (which it surveys of 2,108 users. CIRP's numbers also found that 91 percent of one-year subscribers elect to renew for a second Prime year, while 73 percent of the service's free-trial users decide to pay for Prime.

The survey tracked renewal rates in all three of those cases, dating back as far as the second fiscal quarter of 2014, and it found that 30-day trial upgraders slowed down from June of 2014 until September of last year—which CIRP's study authors attributed to the March 2014 bump in Prime fees from $79 to $99. The rates slowly climbed from that point on, and they've had an upward trend in the other categories, as well—one-year renewals are up from 81 percent in early 2014, and two-year renewals have climbed from 81 percent that year, as well.

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