Well, if you concede that Britney Spears does “typical” pop songs, I checked her YouTube channel and all but two of her videos have running times of between 3 and 4 minutes. Of the two that are longer, one, “Oops! I Did It Again!”, has a running time of 4:11, but the first 39 seconds consist of a “dramatic” bit before the song starts. Three minutes or so has been pretty standard for some time, mainly because radio programmers like 3-minute songs. ‘Weird Al’ Yankovic presents a nice representative sample of “various artist” pop hits over the course of 20+ years, and the overwhelming majority of his songs fall between 3 and 4 minutes, though that’s a bit misleading since he often performs his parodies at a faster tempo than the original and often shortens or entirely leaves out instrumental solos.
Fooling around with iTunes “smart lists” to sort things, I find that 136 of 1239 songs in my library are Japanese pop, or 11%.
632 of the songs in my library run 4 minutes or longer; 119 of these are J-Pop, representing 18.8% of the songs running longer than 4 minutes.
437 of the songs in my library run between exactly 3 minutes and exactly 4 minutes; 14 of these are J-Pop, representing only 3% of the songs in that range.
170 of the songs in my library run less than 3 minutes; 3 of these are J-Pop, representing a mere 1.8% of songs in that range.
J-Pop represents only 2.8% of songs running 4 minutes or shorter in my library, compared to 18.8% of those running longer than 4 minutes.
(percentages are not exact due to rounding)
Granted, there is a certain degree of selection bias here, since my library is limited to the stuff I personally enjoy, and much of the music in my collection is not represented in my iTunes library since I have a lot of stuff on LPs and cassettes that I’ve never gotten around to purchasing on CD. The overall number of songs over 4 minutes is also somewhat skewed due to the fact that my library includes Rush’s complete output. Even so, Japanese pop represents only 11% of my overall collection, but 18.8% of the songs longer than 4 minutes.
However, I have listened to hundreds of Japanese pop songs on YouTube that are not represented in my iTunes library, and the overwhelming majority of these do exceed 4 minutes.