I'm a procrastinator. Of the 35 games I have on hand, I've beaten 30. A few I got over halfway and keep telling myself I'll finish later. A couple I've barely even started, and likely will never finish. If I looked up my whole library of games, I probably beat around 85% of them. About half of those I have 100% completion on. Not the best record, but not bad, much better than 20%. That percent seems really really low to me...
Length doesn't seem to play that big of a factor with my completion. I didn't finish Okami, which was super long, but I have 100% on FF13, which was impossibly long. It's mostly just a matter of whether I hit a boring/difficult patch and have to take a break so that it gets rolled over to procrastination. I got Okami the same time I got Shadow of the Colossus, so when I hit a boring section, I stopped playing and moved to SotC and never got around to going back. Where as FF13 was last on my list after portal 2 and Dragon Age 2, so I had nothing to switch off from it.
Now excuse me while I beat Fire Emblem for the 20th time even though Golden Sun: Dark Dawn is sitting in my gameboy just barely unfinished.