There's a lot of variety in length of a finished video- it depends on what you shoot and what you include. If I do a wedding shoot and spend a full 8-hour day covering pre-ceremony, ceremony and reception, I'm going to end up with a lot more raw footage than I would doing a 3-hour ceremony-only shoot. This will affect the final length.
Also, I've been to ceremonies that last a full hour and the shortest I've been to lasted 12 minutes. Typically, 25 to 35 minutes is normal. If the entire ceremony is included, you'll see a difference in the length of the finished product for this reason as well.
Finally, the package being sold will affect the finished length. If the couple purchases a package with a Love Story, plus a childhood photo montage, several video montages set to music of their choice, along with a honeymoon mantage- well, that's going to be much longer than one with a childhood montage, the ceremony and post-ceremony coverage with no reception.
So the best answer to "How long is a wedding video?" is: as long as you want it to be and as long as the client is willing to pay for.
Cheers!
-Jason