Apple has extended song previews in iTunes from 30 seconds to 90 seconds a few years back for songs longer than two and a half minutes. Obviously, this was a move that pleased users, letting them listen to a big part of the song for free, before deciding whether to purchase or not.
However, now it seems that Apple has started reverting some track preview times back to 30 seconds, the change being reported by several users, on both mobile and desktop versions of iTunes. An official statement hasn't been issued by Cupertino so far. Here's MacRumors is saying:
Not all tracks have been returned back to 30-second previews, and it seems that some of those that have been cut down have been restored to their previous length. This could also be just a glitch, so let us know by leaving your comment below if this works fine for you or not.