Video is generally better short and sweet. You might find 20 mins fascinating but the average joe will switch off after 30 seconds.
Having said that, it depends entirely on both the product and the intended media. For the latter, if you're addressing people already on your site, you can assume a lot more patience than ADD kiddies on YouTube being lured by the kittens in the "more videos" column right next to your video.
Some products are not important purchases for most people. I like the example of cars. You would think that a car, being the most expensive investment most folk ever make, would justify long comprehensive and technical promo videos. Not so - look at any car manufacturer's retail websites, it's all branding and short 2 min lifestyle type nonsense.
On the other hand, we deal in a product which is very niche and people obsess over. We have a (relatively very long) 10 minute promo video on our site which keeps about a 50% completion rate (fairly good we think).