I basically agree with Kev on this. I do agree with the sentiment that a long album (like, over 70 minutes) needs to justify its length... but with the trend toward shorter albums, there are definitely some that I listen to and think "that was it?" I think an album that's under 40 minutes needs to justify its brevity just as much as one that's over 70 needs to justify its length.
For me, the sweet spot is probably 55-65. Right off the top of my head, Images and Words, Century Child and The Mountain are three of my top favorite albums, all albums where I feel that the perfect pacing is part of what makes them so enjoyable, all fall neatly into that range.
I think that the number of songs can also be a factor in ideal length. I think there can be an overload with too many songs just as much as there can be with too long an overall runtime. I'd say the optimal length for a typical pop album is probably shorter, on average, than a typical prog album. If the average song length is, say, 3:30, then an hour-long album is 17 songs. Whereas if the average length is 6:00, an hour-long album is 10 songs. Personally, I would find that 17-song album more tiring than the 10-song album, other things being equal. 17 is just a lot of little pieces to try to focus on.