I'd say 1:30 for a motion graphics reel. But you better hook the person in the first 5 seconds and make them want more once it ends.
You want it long enough to show off your best work, but short enough to make the viewer say to themselves when it is done, "I want more!"
Just like when a close up magic trick is performed. Once it is completed you have a sense of, "show me more!!!"
That is the key of creating a good reel. If there is too much, and too long, it will get boring.
Also, if a person is not hooked in the first 5 seconds of the reel. then it will never be watched to even get to the final 1:30. I see reels all the time on here posted that are too slow to start, or the first couple of graphics are lame, so i turn it off. Sometimes i have gone back to the reel out of boredom and seen great stuff at the end of it. But if i were to hire someone they never would have gotten chosen.
does this make sense?
I guess the best metaphor i can come up with is check out any of the abc stations "Lost" commercials. There is never one of those i see that i am not like, "GIVE ME MORE NOW!!!! I NEED TO SEE THAT EPISODE!!!."
All of our editors/artists have 2 reels: the graphics/ montage reel, which is typically 1:00-1:30 mins, and the before/after or best of reel, which lasts like 2-3 mins. Like everyone says, shorter is better and you better grab the attention within the first 5 seconds...If you're an experienced pro, no longer than 2:30 mins seems good. If you're starting out, please, no more than 1:30.