I think it partially depends on the age of the couple, what their expectations are. The fact this surprise happened seems to indicate not enough communication and mutual understanding of expectations in the planning/ sign-up phase.
Did you not tell them about how long it was going to be, what was going to be left in and left out? This used to be part of my booking “spiel”, describing in detail a list of what I will shoot. Most of my wedding vids wound up at 90 minutes to 2 hours long, including the reception. But I’m an oldster now, with an onion on my belt; I don’t know what these crazy kids expect these days. I suppose in a twitter culture, everything is truncated, including sentimentality and history. Sure, they think they want just a 15 minute montage now, and perhaps you should have led off with that, but I think as they get into a few anniversaries, and later, have kids, they’ll begin to appreciate having more material than just the montage.
I think what I’d do in this case is offer to re-edit a shorter montage, and append it to a DVD so they have the cut they want, plus a “Director’s Cut”. You’ll likely have to eat the extra time this takes, since it wasn’t in writing from the start. The upside is you probably still have the files on the system so there will be little time spent re-digitizing. You might also just import the existing DVD and simply do a cut-down from that, maybe, since you know ALL of those shots are already good.
Best of luck; let us know what shakes out.