This is an important question, but I think it varies by rider. The duration of the climb does make a difference, which of course was the point of the original question. For instance, a 5% incline that only lasts for a few hundred horizontal feet may not even register as a hill in your mind, but a hill that sticks around 5% for 5 miles has a way of dropping you to your lowest gear.

Part of that is that momentum gets you part way up a hill but after that your legs have to do all the work. I think that's actually a small effect though. Physiology plays a bigger role. I don't know the details of how it works, but anyone who has been to a gym is familiar with the idea that it's easier to lift a weight in three sets of 10 with a break in between than it is to lift the same weight 30 times in a row. Muscles need short term recovery.

For this reason, unless I'm looking for a workout, I prefer a series of short hills over one long constant climb, depending on how much steeper the short hills are.