The quote you are looking for is "seventeen months, two weeks, four days, and an odd number of hours...", though Luthor offers a somewhat conflicting timeline in that same issue, indicating that it's been a year.
I've been watching the timeline pretty carefully and organizing it
here, in case you missed it. According to all that I've pieced together, Superman had already been active for nearly 3 years at the time Superman #1 launched. Or, to put it in real-time, he became Superman in 1984, and the current day is 1987.
Really, the only piece of info offered anywhere in any of these issues that has conflicted with any aspect of this timeline is in MoS #5, when Lois makes the comment that she's been waiting 5 years to be kissed by Superman. The idea that Superman had already been active for 5 years by this point is contradicted several times, most notably by the timeline jointly offered by MoS #1 and #6.
And remember, Clark was using his powers anonymously before becoming Superman for 7 years prior to being outed by Lois while saving that space ship. His time in costume has been relatively brief, even though he's now 28 years old and has been away from Smallville since he was 18.
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