Well, that sounds like a question for your institutions legal counsel then.
To me, "the time period in which the current interest rate has been in effect" is talking about the time period since the last time the interest rate was something different. Since you have a floor and the rate has stayed the same, you include the entire period, though it is certainly up for debate. My interpretation seems consistent to me with how a consumer would read the ARM notice. This would be especially true because you would not be providing the consumer with an ARM notice unless the payment changes, so a consumer getting the notice would be confused as to why they didn't get the notice each of the past four years, but again, question for your legal counsel.
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My opinion, take it for what its worth. Opinions expressed are my own and not those of my employer and are not legal advice.