It depends. Too many variables to say. Flat terrain with no interruptions paired with a steady cadence results in few shifts. Stops and terrain changes that as does the riders fitness relative to the terrain. Chainset matters too. With a 53-39 I found that most the time I was in the small ring below 20 mph. False flats and persistent winds kept me there. With favorable winds and the down side of the false flat and it was mostly big ring.
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Sir Mark, Knight of Sufferlandria